The Tenth Amendment of the Constitution of Ireland permitted the state to ratify the Single European Act. It was effected by the Tenth Amendment of the Constitution Act, 1987, which was approved by referendum on 26 May 1987 and signed into law on 22 June of the same year. It is not to be confused with the failed Tenth Amendment Bill of 1986, which was rejected by voters.
Addition to Article 29.4.3 (added text in bold):Tenth Amendment of the Constitution of Ireland Wikipedia
The State may become a member of the European Coal and Steel Community (established by Treaty signed at Paris on the 18th day of April, 1951), the European Economic Community (established by Treaty signed at Rome on the 25th day of March, 1957) and the European Atomic Energy Community (established by Treaty signed at Rome on the 25th day of March, 1957). The State may ratify the Single European Act (signed on behalf of the Member States of the Communities at Luxembourg on the 17th day of February, 1986, and at The Hague on the 28th day of February, 1986).
No provision of this Constitution invalidates laws enacted, acts done or measures adopted by the State necessitated by the obligations of membership of the Communities, or prevents laws enacted, acts done or measures adopted by the Communities, or institutions thereof, from having the force of law in the State.
The Tenth Amendment of the Constitution was the first of a number of amendments that have been made to expressly permit the Irish state to ratify changes to the founding treaties of the European Union (others have been the Eleventh, Eighteenth and Twenty-sixth Amendments). The Tenth Amendment was adopted in response to the ruling of the Supreme Court, in the case of Crotty v. An Taoiseach of the same year, that the constitution, as it stood, did not permit the state to ratify the Single European Act. This was because the Act entailed a diminution of the power of the Government (cabinet) to conduct the nation's foreign policy, a power the constitution explicitly granted to the Government. While the change shown above is that made to the English-language version of the constitution, constitutionally it is the Irish text that takes precedence.
The Tenth Amendment was introduced by the Fianna Fáil government of Charles Haughey and was also supported by Fine Gael, the Progressive Democrats and employers' and farmers' organisations. It was opposed by most members of the Labour Party and the Workers' Party. Voting in the referendum went 755,423 (69.9%) in favour and 324,977 (30.1%) against.