Some 1.2 million people backed the campaign to enshrine rights for same-sex couples in the Constitution, almost two thirds of those who voted.
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.
Leo Varadkar, Health Minister and Ireland's first openly gay cabinet member, described the impact the momentous victory had on the country.
"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.
About 2,000 people gathered in the normally quiet upper courtyard waving pride flags, feather boas and umbrellas as couples embraced and kissed under a huge screen capturing the moment.
Outside the imposing iron gates of the castle, hundreds of others unable to get in joined crowds spilling out from nearby bars to share the moment.
Taoiseach Enda Kenny paid tribute to the 60,000 young people who registered to vote in recent weeks and thousands of emigrants who came home from as far afield as Canada, the US and Australia to cast ballots on Friday.
"It's a piece of history," he said.
The result is all the more significant for the social shift it heralds in a country which was traditionally a bastion of Catholicism and conservative lifestyles.
Diarmuid Martin, Archbishop of Dublin, declared the groundswell of support for same-sex couples was a social revolution that did not happen in the last day.
"I think really the Church needs to do a reality check," the cleric said.
Voters were asked one simple, specific question on whether to amend Article 41 of the 1937 Constitution by adding a new clause to a section titled The Family.
It asked them to support or reject a change to the 78-year-old document which reads: "Marriage may be contracted in accordance with law by two persons without distinction as to their sex."
There was only one blot on the landscape with one of the country's 43 constituencies, Roscommon-South Leitrim opposing the constitutional change by a narrow margin of 51.42% voters against extending marriage rights to gay people.
The biggest support came in Dublin South East where just shy of 75% of voters backed the reform.
Already the resounding victory for gay rights campaigners is being billed as a massive boost for the Republic's reputation on the international stage.
At the same time attention was shifting to Northern Ireland, one of the few regions of Europe were gay marriage is not enshrined in law.
Colm O'Gorman, director of Amnesty International Ireland and a married gay man with two children, said the vote sent a message that all people are cherished and valued.
"It has a great resonance here in Ireland but it's one that's going to echo around the world," he said.
Amnesty revealed plans to ramp up its campaigns for marriage equality in Belfast with director Patrick Corrigan adding: "Northern Ireland is now the last bastion of discrimination against gay people in these islands"
The official result showed almost 1.95m people went to the polls - a higher than normal turnout of 61%.
A large proportion of that was down to students' unions encouraging members to get their names on the electoral register and a spontaneous influx of voting emigrants who marked their return on social media with #hometovote.
Some 734,300 people voted against the proposal, the 34th to be made to the country's constitution which contains seemingly outdated clauses such as a woman's place being in the home.
Senator David Norris, who fought from the 1970s to 1993 to have homosexuality decriminalised, said it was a wonderful result.
"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," he said.
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."
Grainne Healy, co-director of the Yes Equality group, said: "Today's result means that having been 'branded and isolated' for decades, each lesbian and gay person knows now that they too belong in Ireland, as full, equal citizens."
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."
Joan Burton, Tanaiste (deputy prime minister) and Labour Party leader, said Ireland had chosen to create a more compassionate and egalitarian constitution.
"When I think back on this campaign, I'll think of many things," she said.
"I'll think of the people who made it happen, and those whose lives will be forever changed.
"I'll think of the joyous reality that when the time came, our country stood up for its people - all its people - and said Yes to equality."
The Labour Party spearheaded political calls for the referendum after going into coalition government in 2011.
Ms Burton added: "But most of all, I'll think of the children.
"The children in every town, village and schoolyard who will now grow up knowing their country accepts them - whoever it is they one day grow to be, and whoever it is they one day grow to love."
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