PROTHENA CORP PUBLIC LTD CO

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Check the appropriate box:

PROTHENA CORPORATION PUBLIC LIMITED COMPANY

(Name of registrant as specified in its charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

PROTHENA CORPORATION PLC

Registered in Ireland - No. 518146

77 Sir John Rogerson’s Quay, Block C, Grand Canal

Docklands, Dublin 2, D02 VK60, Ireland

NOTICE OF ANNUAL GENERAL MEETING OF

SHAREHOLDERS TO BE HELD ON MAY 13, 2025

To the Shareholders of Prothena Corporation plc:

NOTICE IS HEREBY GIVEN that the Annual General Meeting of Shareholders (the “Annual Meeting”) of Prothena Corporation plc, an Irish public limited company (the “Company”), will be held on Tuesday, May 13, 2025, at 4:00 p.m. local time, at The Merrion Hotel, Upper Merrion Street, Dublin 2, D02 KF79, Ireland for the following purposes:

1. To elect, by separate resolutions, Paula K. Cobb, Lars G. Ekman, and Gene G. Kinney, as directors, to hold office until no later than the annual general meeting of shareholders in 2028;

2. To ratify, in a non-binding vote, the appointment of KPMG LLP as the Company’s independent registered public accounting firm for its fiscal year ending December 31, 2025, and to authorize, in a binding vote, the Company’s Board of Directors, acting through its Audit Committee, to approve the remuneration of that auditor;

3. To approve, in a non-binding advisory vote, the compensation of the Company’s executive officers named in the Proxy Statement accompanying this Notice;

4. To approve an amendment to the Company’s 2018 Long Term Incentive Plan to increase the number of ordinary shares available for issuance under that Plan by 2,000,000 ordinary shares; and

5. To transact such other business as properly comes before the Annual Meeting or any adjournment or postponement thereof.

The foregoing items of business are more fully described in the Proxy Statement accompanying this Notice. The Company’s Irish statutory financial statements for its fiscal year 2024, including the reports of the directors and auditors thereon, will be presented at the Annual Meeting. There is no requirement under Irish law that those statutory financial statements be approved by shareholders and no such approval will be sought at the Annual Meeting. The Annual Meeting will also include a review of the Company’s affairs.

Under our Constitution and Irish law, Proposal Nos. 1, 2, 3, and 4 are deemed to be ordinary resolutions requiring the approval of a simple majority of the votes cast at the Annual Meeting.

Only shareholders who owned our ordinary shares at the close of business on March 3, 2025, may vote at the Annual Meeting. Each shareholder of record will be entitled to one vote per ordinary share on each matter submitted to a vote of the shareholders, as long as those shares are represented at the Annual Meeting, either in person or by proxy. Shareholders who are entitled to attend and vote at the Annual Meeting are entitled to appoint a proxy or proxies to attend and vote on their behalf at the Annual Meeting; such proxy is not required to be a shareholder of the Company.

Our Board of Directors unanimously recommends that you vote as follows on the matters to be presented to shareholders at the Annual Meeting:

1. FOR the election of Paula K. Cobb, Lars G. Ekman, and Gene G. Kinney, as directors, as described in Proposal No. 1;

2. FOR the ratification, in a non-binding vote, of the appointment of KPMG LLP as the Company’s independent registered public accounting firm for its fiscal year 2025, and authorization, in a binding vote, of the Board of Directors, acting through its Audit Committee, to approve the remuneration of that auditor, as described in Proposal No. 2;

3. FOR the approval, in a non-binding advisory vote, of the compensation of the Company’s named executive officers, as described in Proposal No. 3; and

4. FOR the approval of the amendment to the Company’s 2018 Long Term Incentive Plan to increase the number of ordinary shares available for issuance under that Plan by 2,000,000 ordinary shares, as described in Proposal No. 4.

For the Annual Meeting, we have elected to use the internet as the primary means of providing our proxy materials to shareholders. Consequently, some shareholders may not receive paper copies of our proxy materials. We intend to send shareholders a Notice of Internet Availability of Proxy Materials with instructions for accessing the proxy materials and for voting via the internet. The Notice of Internet Availability of Proxy Materials will also provide the date, time, and location of the Annual Meeting; the matters to be acted upon at the meeting and the Board of Directors’ recommendation with regard to each matter; a toll-free number, an e-mail address, and a website where shareholders can request a paper or e-mail copy of our Proxy Statement and form of proxy card and our Annual Report on Form 10-K for fiscal year 2024; information on how to access their proxy card; and information on how to attend the meeting and vote in person.

You are cordially invited to attend the Annual Meeting, but whether or not you expect to attend in person, you are urged to complete, sign, and date your proxy card and return it by mail or follow the alternative voting procedures described in the Notice of Internet Availability of Proxy Materials or the proxy card.

By Order of the Board of Directors

Yvonne M. Tchrakian

Company Secretary

Dublin, Ireland

March 28, 2025