In a historic referendum, more than 62 per cent voted "yes" to amend the country's constitution to allow gay and lesbian couples to marry.
The huge "Yes" vote marks a milestone in Ireland's journey towards a more liberal, secular society.
Only one of Ireland's 43 constituencies, the central Roscommon-South Leitrim voted against the constitutional change.
Irish Prime Minister Enda Kenny said it was a "small country with a big message for equality" around the world.
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The referendum was held 22 years after homosexual acts were decriminalised in the Republic of Ireland and five years after parliament approved marriage-style civil partnerships for gay couples.
Same-sex marriage is now legal in 20 countries worldwide.
Out of an electorate of more than three million, some 1.2 million people backed gay marriage, almost two thirds of those who voted. 734,300 voters said No.
New laws on gay marriage will be put to the Dail parliament before the summer potentially paving the way for the first ceremonies to take place before the end of the year.
"Something has been awakened in the Irish people... It was not just a referendum it was more like a social revolution," he said.
In stirring and emotional scenes in the grounds of Dublin Castle, the near two to one majority was officially declared shortly before 7pm, sparking tears and joy.
But the unexpected strong percentage of "yes" voters surprised both sides.
Analysts and campaigners credited the "yes" side with adeptly using social media to mobilise first-time young voters.
The pro-reform vote was also energised by an 11th-hour movement called #hometovote, which used social media to encourage young Irish expatriates to get back to Ireland in time to vote.