The Republic of Ireland becomes the first nation in the world to utilise a public referendum to legalise gay marriage.

Ireland, also known as the Republic of Ireland, is a country in Northwestern Europe. It consists of 26 of the 32 counties of the island of Ireland, with a population of about 5.4 million. Its capital and largest city is Dublin, on the eastern side of the island, with a population of over 1.5 million. The sovereign state shares its only land border with Northern Ireland, which is part of the United Kingdom. It is otherwise surrounded by the Atlantic Ocean, with the Celtic Sea to the south, St George's Channel to the south-east and the Irish Sea to the east. It is a unitary, parliamentary republic. The legislature, the Oireachtas, consists of a lower house, Dáil Éireann; an upper house, Seanad Éireann; and an elected president who serves as the largely ceremonial head of state, but with some important powers and duties. The head of government is the Taoiseach, elected by the Dáil and appointed by the president, who appoints other government ministers.

The government of Ireland held referendums on 22 May 2015 on two proposed amendments to the Constitution of Ireland which had been recommended by the Constitutional Convention. The amendment to permit same-sex marriage in the Republic of Ireland was approved by 62–38% of the voters. The other amendment would have reduced the age of candidacy for the President of Ireland from 35 to 21, but voters rejected it by 73–27%. A Dáil by-election in Carlow–Kilkenny was held on the same day. Other amendments were considered but not proceeded with, including reducing the voting age from 18 to 16, and sanctioning the establishment of a Unified Patent Court.