Voting has begun in Ireland as campaigners bid to make it the first country in the world to introduce gay marriage by popular vote.
Twenty-one countries have already extended the right to same-sex couples, but more than 3.2 million voters in the Republic have been asked for their say, with polling stations open from 7am to 10pm.
The country has seen a surge in voter registration in recent weeks - 66,000 mostly young people and students - while in the last 24 hours social media documented many emigrants, mostly from the UK and Europe, returning home to cast their ballots.
Among the electorate are 400,000 18-25-year-olds, who Yes campaigners believe will be key to a successful passing of the proposal.
Voters are being asked one simple, specific question on whether to amend Article 41 of the 1937 Constitution by adding a new clause to a section titled The Family.
It asks voters to support or reject a change to the 78-year-old document which reads: “Marriage may be contracted in accordance with law by two persons without distinction as to their sex.”
It does not suggest any change to the definition of the family or remove any outdated references in the section, including those that state a woman’s place is at home.
If passed, it will be the 34th constitutional amendment - and only 22 years since Ireland decriminalised homosexuality.
Among those promoting support for gay marriage on Twitter was Taoiseach Enda Kenny, who said: “For gay sons and daughters, brothers and sisters, family and friends, Yeats said it best: Tread softly because you tread on my dreams.”
More than 2,000 islanders eligible to vote headed to polling stations off Donegal, Mayo and Galway yesterday, maintaining a tradition of early offshore polling days for fear that bad weather will disrupt the counts.
Other countries have held referendums on gay marriage, including Croatia, Slovakia and Slovenia were the extension of the right was not passed by the electorate.
The counting of ballots starts in the 43 constituencies at 9am tomorrow, and a picture of how tight the contest has been should emerge by midday.
Opinion polls put the Yes side well in front until a week ago, but concerns have been growing about undecided voters - around a quarter of those polled declined to declare their intentions.
The Yes campaign has been backed by all the main political parties but a small number of backbenchers are to vote no.
Concerns have grown that the more conservative politicians from the Fianna Fail and Fine Gael parties did not sufficiently canvass support, particularly in rural areas, amid fears of a backlash at next year’s general election and a sense of complacency that the proposal would be accepted.
In a separate referendum also being held today, voters are being asked their views on proposals to reduce the age limit on who can stand for the presidency from 35 to 21.