The result makes Ireland, where 85 percent of people are Catholic, the first country in the world to legalize same-sex marriage through a popular vote.
Unofficial figures indicated the referendum saw the highest turnout for an Irish constitutional referendum since 1996, when the Irish Republic narrowly voted to lift a ban on divorce.
Ireland's president, Michael D. Higgins, said he could not comment on the outcome of the referendum but welcomed the high voter turnout.
Ultimately, all but one of Ireland's 43 electoral districts vote in favor of the amendment. In Roscommon-South Leitrim, the only district to oppose legalizing same-sex marriage, the Yes and No camps were separated by a little more than 1000 votes.
Same-sex couples in Ireland have been able to enter into civil partnerships since 2011, but Friday's referendum-where all the main political parties supported a Yes vote-was held to ensure such unions would be recognized by the constitution.
Irish Prime Minister Enda Kenny said "it's certainly better than it was yesterday" when asked whether Ireland was the "best small country in which to be gay," the Irish Times reported.
The No campaign, backed by Ireland's Catholic Church and other conservative groups, had focused on family values. One poster had argued children deserved both a mother and a father, while another pointed to civil partnerships as a reason why there was no need to redefine marriage.