The Thirty-fourth Amendment of the Constitution Act 2015 amended the Constitution of Ireland to permit marriage to be contracted by two persons without distinction as to their sex. Prior to the enactment, the Constitution was assumed to contain an implicit prohibition on same-sex marriage in the Republic of Ireland. It was approved at a referendum on 22 May 2015 by 62% of voters on a turnout of 61%. This was the first time that a state legalised same-sex marriage through a popular vote. Two legal challenges regarding the conduct of the referendum were dismissed on 30 July by the Court of Appeal, and the bill was signed into law by the President of Ireland on 29 August. An amendment to the Marriage Act 2015 provided for marriages permitted by the new constitutional status. The act came into force on 16 November 2015; the first same-sex marriage ceremony was held on 17 November 2015.

The Constitution of Ireland is the fundamental law of Ireland. It asserts the national sovereignty of the Irish people. The constitution, based on a system of representative democracy, is broadly within the tradition of liberal democracy. It guarantees certain fundamental rights, along with a popularly elected non-executive president, a bicameral parliament, a separation of powers and judicial review.