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If you are in any doubt as to the action to be taken, you should consult with your independent financial adviser who, if you are taking advice in Ireland, is authorised or exempted under the European Communities (Markets in Financial Instruments) Regulations 2017 (as amended) or the Investment Intermediaries Act, 1995 (as amended).

If you have sold or transferred your entire holding of ordinary shares in ICON, please pass this document, together with the attached proxy form, to the purchaser or transferee, or to the stockbroker, bank or other agent through whom the sale was effected, for transmission to the purchaser or transferee as soon as possible.

The purpose of this letter is to outline the background to and summarise the resolutions to be proposed at the AGM. Please refer to the form of proxy for the AGM (which is separately enclosed) and the notes on pages 8 to 10 for details on how to vote your shares and return your form of proxy. Your attention is also drawn to the notice of the AGM on pages 5 to 7 which sets out the matters to be considered at the AGM.

In accordance with our Constitution (by-laws), one third of the members of the Board who are subject to retirement by rotation are required to retire from office and may stand for re-election at the AGM. However, in July 2022 in response to stakeholder feedback and current corporate governance best practice, the Board of Directors unanimously agreed that all of the Directors will retire and stand for re-election annually at each AGM. Accordingly, the following Directors (being all of the Directors) will stand for re-election:

Mr. Ciaran Murray, Mr. Rónán Murphy, Dr. John Climax, Mr. Eugene McCague, Ms. Joan Garahy, Ms. Julie O’Neill and Dr. Linda Grais are “independent” non-executive Directors (as that term is defined under the published listing requirements of NASDAQ). Dr. Steve Cutler, ICON’s Chief Executive Officer, is an executive officer.

Each of the Directors standing for re-election demonstrates the necessary commitment to the role and provides valuable skills, knowledge and experience and makes important contributions to the working of the Board. Further information on the experience, qualifications and industry knowledge of the current Directors is available from the Annual Report and/or Form 20-F at https://investor.iconplc.com/financials-filings/annual-reports and the details of current committee composition is set out on our website at https://investor.iconplc.com/corporate-governance/committee-composition.

This resolution authorises the Directors to issue shares until the earlier of the next Annual General Meeting of the Company or 24 January 2025 up to an aggregate of 20% of the share capital of the Company without further shareholder approval.

This resolution is required under Irish law as the Company is an Irish incorporated company. The 20% cap on this resolution aligns the resolution with the NASDAQ rules which provide that up to 20% of share capital can be issued without shareholder approval.

This resolution authorises the Directors to issue shares for cash, subject to resolution 4, until the earlier of the next Annual General Meeting of the Company or 24 January 2025 up to an aggregate of 20% of the share capital of the Company without having to offer the shares to existing shareholders on a pro rata basis.

A resolution authorising the issuance of shares for cash without such offer round is a requirement of Irish law and there is no such requirement under the applicable U.S. Securities and Exchange Commission (the SEC) rules and NASDAQ rules. Accordingly, approval of this resolution would merely place us on a par with other NASDAQ-listed companies and will facilitate our ability to fund acquisitions and otherwise raise capital. As above, this year the authority under this resolution expires on the earlier of the next Annual General Meeting of the Company or 24 January 2025.

The caps on resolutions 4 and 5 provide our shareholders with anti-dilution protection equivalent to protections provided to the shareholders of many other NASDAQ-listed companies. Every year since the Company was listed (apart from 2014 as resolutions with 5 year authority were passed in 2013), the Directors have put resolutions to authorise the issuance of shares and to disapply offer round to the shareholders and each such resolution has been passed.

This resolution authorises the Company to purchase in the market (buy-back) up to 10% of the outstanding share capital of the Company. It is important both for the Company and shareholders that the Company has this flexibility to implement a buy-back (without having to seek further shareholder approval) if the market conditions favour a buy-back. It should also be noted that while the applicable SEC rules and NASDAQ rules do not require shareholder approval prior to a share buy-back, this resolution is required as the Company is an Irish incorporated company and Irish law requires shareholders to pass such a resolution to give Directors the authority to put a buy-back in place. The authority under this resolution expires on the earlier of the next Annual General Meeting and 24 January 2025.

This resolution authorises the price range at which the Company can reissue shares that it holds as treasury shares. Any share buy-back activity by the Company will result in ordinary shares either being cancelled or reissued as treasury shares. We may reissue treasury shares that we acquire through our proposed share buy-back activities including in connection with our executive compensation programme, our employee restricted share unit programme and our other compensation programmes. As a result of using this authority in this way, ICON would avoid the need to issue new shares (and the resulting shareholder dilution) when the vesting of equity awards triggers the requirement to issue shares.

The authority being sought from shareholders provides that the minimum and maximum prices at which an ordinary share held in treasury may be reissued are 95% and 120%, respectively, of the average closing price per ordinary share of the Company, as reported by NASDAQ, for the thirty (30) trading days immediately preceding the proposed date of reissuance. Any reissuance of treasury shares will be at price levels that the Board considers in the best interests of our shareholders.

There is no resolution dealing with executive compensation as ICON, being a foreign private issuer, is not obliged to provide a “say on pay” shareholder resolution on executive compensation. Details of ICON’s executive officers' compensation for 2022 are available in the Annual Report and Form 20-F for 2022 which are available at https://investor.iconplc.com/financials-filings/annual-reports.

Resolutions 1.1 to 1.8, 2, 3 and 4 are ordinary resolutions which require approval of a simple majority of the votes cast in person or by proxy and resolutions 5 to 7 are special resolutions which require approval of 75% of the votes cast in person or by proxy.

Your Board believes that the resolutions to be proposed at the AGM are in the best interests of the Company and its shareholders. Accordingly, your Directors unanimously recommend that you vote in favour of all resolutions as they intend to do in respect of the shares held by them. On 2 June 2023, the Directors held 602,561 ordinary shares representing approximately 0.73% of the issued ordinary share capital of the Company.

4.“That the Directors be and are hereby generally and unconditionally authorised, pursuant to Section 1021 of the Companies Act 2014, to exercise all the powers of the Company to allot relevant securities (within the meaning of Section 1021 of the Companies Act 2014) up to an aggregate nominal amount of €984,882.47 representing approximately 20% of the aggregate nominal value of the issued ordinary share capital of the Company as at 7 June 2023 and the authority conferred by this resolution shall expire on the earlier of the date of the next Annual General Meeting of the Company or 24 January 2025, unless previously renewed, varied or revoked; provided that the Company may make an offer or agreement before the expiry of the authority conferred by this resolution, which would or might require any such securities to be allotted after the authority conferred by this resolution has expired and, in that case, the Directors may allot relevant securities in pursuance of any such offer or agreement as if the authority conferred hereby had not expired.

5.“That, subject to the passing of Resolution 4, the Directors be and are hereby empowered pursuant to Section 1022 and Section 1023(3) of the Companies Act 2014, to allot equity securities (as defined in Section 1023 of the Companies Act 2014) for cash as if the provisions of sub-section (1) of the said Section 1022 did not apply to any such allotment up to an aggregate nominal amount of €984,882.47 representing approximately 20% of the aggregate nominal value of the issued ordinary share capital of the Company as at 7 June 2023 and the authority conferred by this resolution shall expire on the earlier of the date of the next Annual General Meeting of the Company or 24 January 2025, unless previously renewed, varied or revoked; provided that the Company may make an offer or agreement before the expiry of the authority conferred by this resolution, which would or might require any such securities to be allotted after the authority conferred by this resolution has expired and, in that case, the Directors may allot relevant securities in pursuance of any such offer or agreement as if the authority conferred hereby had not expired.”

6.“That the Company and/or any subsidiary (as such expression is defined by Section 7 of the Companies Act 2014) of the Company be and they are hereby generally authorised to make overseas market purchases (as defined by Section 1072(2) of the Companies Act 2014) of shares of any class of the Company on such terms and conditions and in such manner as the Directors or, as the case may be, the Directors of such subsidiary, may from time to time determine in accordance with and subject to the provisions of the Companies Act 2014 and the following restrictions and provisions:

(i)The maximum aggregate number of shares authorised to be acquired pursuant to this resolution shall not exceed 10% of the aggregate number of shares issued by the Company at close of business on the date of passing of this resolution;

(iii)The maximum price (exclusive of expenses) to be paid for any ordinary share shall be an amount equal to 115% of the NASDAQ Official Close Price (the “NOCP”) (as reported by NASDAQ) of the Company’s ordinary shares on the trading day preceding the day on which the relevant shares are purchased by the Company.

The authority hereby conferred shall expire on the earlier of the date of the next Annual General Meeting of the Company or 24 January 2025 or (if earlier) unless previously varied, revoked or renewed in accordance with the provisions of Section 1074 of the Companies Act 2014. The Company or any subsidiary may before such expiry make a contract for the purchase of shares which would or might be wholly or partly executed after such expiry and may make a purchase of shares pursuant to any such contract as if the authority hereby conferred had not expired.”

(ii) the minimum price at which a treasury share may be reissued off-market shall be the nominal value of the share where such a share is required to satisfy an obligation under an employee share plan operated by the Company or, in all other cases, an amount equal to 95% of the “market price”; and

(iii) for the purposes of this resolution, the “market price” shall mean the average closing price per ordinary share of the Company, as reported by NASDAQ, for the thirty (30) trading days immediately preceding the proposed date of reissuance.

The authority hereby conferred to reissue treasury shares shall expire eighteen months from the date of the passing of this resolution unless previously varied or renewed in accordance with the provisions of Section 1078 of the Companies Act 2014.”

Information regarding the Annual General Meeting is available on the Company’s website www.iconplc.com and from www.proxyvote.com. If you require a paper copy of the Form 20-F or Annual Report, please contact Investor Relations at 1-888-381-7923 or IR@iconplc.com.

If your shares are registered in your name, you are a shareholder of record. Shareholders of record who are entered in the Register of Members of the Company on 2 June 2023 shall be entitled to attend, speak, ask questions, vote, demand a poll, and/or join in the demand for a poll at the Annual General Meeting, or if relevant, any adjournment thereof. Changes in the Register of Members of the Company after that time will be disregarded in determining the right of any person to attend and/or vote at the Annual General Meeting.

For those shareholders whose shares are not held in their name, but rather in an account at a brokerage firm, bank, dealer or other similar organisation, who in turn hold through The Depository Trust Company (DTC), then their entitlement to vote is determined as at 2 June 2023.

As a shareholder of record, you may vote in person at the Annual General Meeting or vote by proxy. In the case of joint holders, the vote of the senior member who tenders a vote, whether in person or by proxy, shall be accepted to the exclusion of the votes of the other shareholder of record and, for this purpose, seniority shall be determined by the order in which the names stand in the Register of Members of the Company in respect of the joint holding. The appointment of a proxy will not preclude a shareholder of record from attending, speaking, asking questions and voting at the Annual General Meeting should the shareholder subsequently wish to do so. A proxy need not be a member of the Company. If you wish to appoint more than one proxy or a person not listed on the form of proxy, please contact Investor Relations at 1-888-381-7923 or IR@iconplc.com.

A Form of Proxy is enclosed with this notice of Annual General Meeting for shareholders of record. To be effective, the Form of Proxy duly completed and executed, together with any authority under which it is executed, or a copy thereof certified, must be deposited at the registered office of the Company, so as to be received by 11.59pm ET on 24 July 2023 or if the Annual General Meeting is adjourned, on the day that falls before the day appointed for the adjourned meeting or (in the case of a poll taken otherwise than at or on the same day as the Annual General Meeting or adjourned meeting) the day before the taking of the poll at which it is to be used. Any alteration to the Form of Proxy must be initialled by the person who signs it.

Alternatively, provided it is received by 11.59pm ET on 23 July 2023 or if the Annual General Meeting is adjourned, by 11.59pm ET on the day that falls 48 hours before the time appointed for the adjourned meeting or (in the case of a poll taken otherwise than at or on the same day as the Annual General Meeting or adjourned meeting) by 11.59pm ET on the day that falls 48 hours before the taking of the poll at which it is to be used, the appointment of a proxy may be submitted by telephone or electronically, subject to the applicable terms and conditions, using the phone number on the Form of Proxy and following the instructions provided or via the Internet by accessing Broadridge’s website www.proxyvote.com and following the instructions on the website. The information you need to appoint your proxy by telephone or electronically is included at the top of your Form of Proxy.

You need only vote in one way (so that, if you vote by Internet or by telephone, you need not return the Form of Proxy). In the case of a corporation, the Form of Proxy must be either executed under seal or signed on its behalf by a duly authorised officer or attorney.

If, as at 2 June 2023, your shares were not held in your name, but rather in an account at a brokerage firm, bank, dealer or other similar organisation, who in turn hold through DTC, then you are the beneficial owner of shares held in “street name” and these proxy materials are being forwarded to you by that organisation, together with instructions as to voting. You will need to carefully follow the instructions from your broker, bank or other agent or contact your broker, bank or other agent if you have any queries.

As a beneficial owner, you have the right to direct your broker or other agent on how to vote the shares in your account as per the instructions enclosed by your broker. You are also invited to attend the Annual General Meeting. However, since you are not the shareholder of record, you may not vote your shares in person at the Annual General Meeting unless you contact your broker and obtain a valid proxy card from your broker or other agent.

Therefore as a beneficial owner of shares registered in the name of your broker, bank or other agent, who in turn hold the shares through DTC, you should have received a voting instruction card and voting instructions with these proxy materials from that organisation rather than from us. Simply follow the instructions on the voting instruction card provided by your broker, bank or other agent to ensure that your vote is counted.

The total number of issued ordinary shares on the record date, 2 June 2023, was 82,017,264. On a vote on a show of hands, every shareholder present in person and every proxy has one vote (but no individual shall have more than one vote). On a poll, every shareholder present in person and every proxy shall have one vote for every share carrying rights of which he is the holder or proxy. Ordinary resolutions are required to be passed by a simple majority of shareholders voting in person or by proxy. Special resolutions are required to be passed by a majority of 75 per cent of shareholders voting in person or by proxy.

If your shares are held by a broker on your behalf (that is, in “street name”), and you do not instruct the broker as to how to vote these shares, the broker may not exercise discretion to vote for or against any of the proposals. This would be a “broker non-vote” and these shares will not be counted as having been voted on the proposals. Please instruct your bank or broker so your vote can be counted.

•You may submit another properly completed proxy with a later date. Your revised proxy must be received before the commencement of the Annual General Meeting at 09.30am Dublin time on 25 July 2023 or if the Annual General Meeting is adjourned, before the commencement of the adjourned meeting;

•You may send a written notice that you are revoking your proxy to Erina Fox, Deputy Company Secretary, ICON plc at the registered office of the Company (being South County Business Park, Leopardstown, Dublin 18, Ireland) or by email to IR@iconplc.com. Your notice must be received before the commencement of the Annual General Meeting at 09.30am Dublin time on 25 July 2023 or if the Annual General Meeting is adjourned, before the commencement of the adjourned meeting; or

If you receive more than one set of materials, your shares are registered in more than one name or are registered in different accounts. In order to vote all the shares you own, you must sign and return all of the proxy cards or follow the instructions for any alternative voting procedure on each of the proxy cards you receive.