The country becomes the first in the world to introduce same-sex marriage after a national referendum, with 62 per cent voting yes.
There were jubilant scenes at Dublin Castle just before 7pm as the official result was declared to a crowd of thousands.
Campaigners had declared victory hours earlier on Saturday as early results showed that a clear majority of voters were backing constitutional reform.
The final result showed that 62 per cent – just over 1.2 million people – voted yes, and around 734,300 people voted against the proposal.
More than 60 per cent of eligible voters had cast their ballot, the highest turnout at an Irish referendum for more than two decades.
All but one of the country’s 43 constituencies voted yes. Voters in Roscommon-South Leitrim opposed the constitutional change by a narrow margin of 51.42 per cent of voters who were against extending marriage rights to gay people.
The biggest support for a yes vote came in Dublin South East where just under 75 per cent of voters backed the reform.
New laws on gay marriage will be put to the Dail before the summer and the first same-sex ceremonies will take place before the end of the year.
Voters were asked whether to amend Article 41 of the country’s constitution by adding a new clause to a section titled The Family: “Marriage may be contracted in accordance with law by two persons without distinction as to their sex.”
Marriage may be contracted in accordance with law by two persons without distinction as to their sex. Proposed amendment to Irish constitution
It was the 34th amendment to the Irish constitution and would come 22 years since Ireland decriminalised homosexuality.
Taoiseach Enda Kenny paid tribute to the 60,000 young people who registered to vote in recent weeks and thousands of emigrants who came back to Ireland to vote on Friday, calling the poll “a piece of history”.
Senator David Norris, who fought to have homosexuality decriminalised, said:”We’ve been brought on board as equal citizens by the generosity of and decency of our straight and fellow citizens and for that I am deeply grateful.”
Church of Ireland bishops said: “We would now sincerely urge a spirit of public generosity, both from those for whom the result of the referendum represents triumph, and from those for whom it signifies disaster.”
Mothers and Fathers Matter, a group which campaigned for a No vote, said: “From our point of view, we have represented a proportion of the population greater than those who support any political party.
“One in three Irish people in this campaign was not represented by the political establishment, the media or the institutions of the state.”
Diarmuid Martin, Archbishop of Dublin, said: “It’s a social revolution that’s been going on – perhaps in the church people have not been as clear in understanding what that involved.
“It’s clear that if the referendum is an affirmation of the views of young people the church has a huge task in front of it.”
Catholic bishops have issued open letters to congregations outlining their concerns about gay marriage and why the church would not support the reform.
But the archbishop told RTE: “I think really the Church needs to do a reality check.
“I appreciate how gay and lesbian men and women feel on this day. That they feel this is something that is enriching the way they live. I think it is a social revolution.
“We have to stop and have a reality check, not move into denial of the realities. We won’t begin again with a sense of renewal with a sense of denial.”
He added: “Most of these young people who voted yes are products of our Catholic school system for 12 years. I’m saying there’s a big challenge there to see how we get across the message of the church.”
Sinn Fein president Gerry Adams said: “It’s hugely important and I want to thank everyone who voted but I especially want to thank all those who voted ‘tá’ – who voted yes – and I want to commend all the equality campaigners.
“It was a wonderful campaign to be part of. They were enthusiastic, idealistic, happy and very, very positive. So I think it’s a huge day for equality.”