As the official ballot counting continued, the only question appeared to be how large the "yes" margin of victory from Friday's vote would be. Analysts said the "yes" support was likely to exceed 60 percent nationally when official results are announced later Saturday.
Gay couples hugged and kissed each other amid scenes of jubilation at counting centers and at the official results center in Dublin Castle, whose cobblestoned central square was opened so thousands of revelers could sit in the sunshine and watch the results live on big-screen televisions.
PHOTOS: Ireland votes to legalize gay marriage
"We're the first country in the world to enshrine marriage equality in our constitution and do so by popular mandate. That makes us a beacon, a light to the rest of the world, of liberty and equality. So it's a very proud day to be Irish," said Leo Varadkar, a Cabinet minister who came out as gay at the start of a government-led effort to amend Ireland's conservative Catholic constitution.
"People from the LGBT (lesbian, gay, bisexual and transgender) community in Ireland are a minority. But with our parents, our families, or friends and co-workers and colleagues, we're a majority," said Varadkar, who watched the votes being tabulated at the County Dublin ballot center.
"For me it wasn't just a referendum. It was more like a social revolution," he said.
In the first official result, the Dublin North West constituency voted 70.4 percent "yes" to gay marriage. But the outcome was already beyond dispute as observers, permitted to watch the paper ballots being counted at all election centers, offered precise tallies giving the "yes" side an unassailable nationwide lead.
Michael Barron and Jaime Nanci, a gay couple legally married in South Africa five years ago, celebrated with friends at the Dublin City counting center as the reality sank in that, once Ireland's parliament passes the complementary legislation, their foreign marriage will be recognized in their homeland.
"Oh.My.God! We're actually Married now!" Nanci tweeted to his spouse and the world, part of a cavalcade of tweets from Ireland tagged #LandslideOfLove.
Political analysts who have covered Irish referendums for decades agreed that Saturday's emerging landslide marked a stunning generational shift from the 1980s, when voters still firmly backed Catholic Church teachings and overwhelmingly voted against abortion and divorce.
"We're in a new country," said political analyst Sean Donnelly, who called the result "a tidal wave" that has produced pro-gay marriage majorities in even the most traditionally conservative rural corners of Ireland.
"I'm of a different generation," said the gray-haired Donnelly, who has covered Irish politics since the 1970s. "When I was reared up, the church was all powerful and the word 'gay' wasn't even in use in those days. How things have moved from my childhood to now. It's been a massive change for a conservative country."
Ireland's deputy prime minister, Labour Party leader Joan Burton, said Ireland was becoming "a rainbow nation with a huge amount of diversity." She said while campaigning door to door, she met older gay people who described how society made them "live in a shadow and apart," and younger voters who were keen to ensure that Irish homosexuals live "as free citizens in a free republic."
The "yes" side ran a creative, compelling campaign that harnessed the power of social media to mobilize young voters, tens of thousands of whom voted for the first time Friday. The vote came five years after parliament approved marriage-style civil partnerships for gay couples.
Those seeking a "no" outcome described their defeat as almost inevitable, given that all of Ireland's political parties and most politicians backed the legalization of homosexual unions.
David Quinn, leader of the Catholic think tank Iona Institute, said he was troubled by the fact that no political party backed the "no" cause.
"We helped to provide a voice to the hundreds of thousands of Irish people who did vote no. The fact that no political party supported them must be a concern from a democratic point of view," he said.
Fianna Fail party leader Michael Martin, a Cork politician whose opposition party is traditionally closest to the Catholic Church, said he couldn't in good conscience back the anti-gay marriage side because "it's simply wrong in the 21st century to oppress people because of their sexuality."
Some in Martin's party - the perennial heavyweight in Irish politics but decimated since its ouster from power following Ireland's 2010 international bailout - did privately oppose the amendment, but only one spoke out in favor of the "no" side.
John Lyons, one of just four openly gay lawmakers in the 166-member parliament, waved the rainbow flag of the Gay Pride movement in the Dublin City counting center and cried a few tears of joy. He paid special credit to the mobilization of younger voters, many of whom traveled home from work or studies abroad to vote.
"Most of the young people I canvassed with have never knocked on a door in their lives," Lyons said. "This says something about modern Ireland. Let's never underestimate the electorate or what they think."