Out of Ireland's 43 constituencies, 40 have voted "yes," the Irish public broadcaster RTE reported on Saturday.
More than 62 percent voted in favor of legalizing same-sex marriage, 37.7 percent voted against the bill.
Strong leads for the "Yes" camp emerged in districts across Dublin, as well as in traditionally conservative rural areas.
But one constituency, Roscommon-South Leitrim, in the middle of the country, saw over 51 percent of voters there rejecting the marriage referendum proposal - a higher percentage than anywhere else.
Thousands in Dublin to celebrate
Large crowds flocked to central Dublin to celebrate the victory.
The crowds cheered as gay rights icons appeared on the square, including David Norris, whose campaign led to the 1993 decriminalization of homosexuality and Rory O'Neill, whose Panti Bliss drag queen character became the face of the campaign, the news agency Reuters reports.
Leo Varadkar, Ireland's health minister, who has come out as gay said: "People from the [gay] community in Ireland are a minority, but with our parents, our families, or friends and co-workers and colleagues, we're a majority. For me, it wasn't just a referendum. It was more like a social revolution," the news agency AP quotes him as saying.
Gay senator David Norris hails the crowds at Dublin Castle.
Nicola Sturgeon, the leader of the Scottish National Party, also expressed her happiness over the Irish victory.
Reduced impact of Catholic Church
The Catholic Church once controlled virtually every aspect of Irish life, but its clout has been vastly reduced by the impact of secularization and a wave of child sex abuse scandals that discredited the clergy.
With the referendum Ireland becomes the 19th country in the world to recognize marriage equality and the 14th in Europe - as well as the first to do so by popular vote. In Germany, same-sex partners have the opportunity to register a partnership, but they do not have the right to marry.
Saturday's referendum results are a stunning triumph for gay rights activists. It was the culmination of a four-decade struggle for gay rights.
ra/ng (AP, Reuters, dpa)