Correction: An earlier version of this video incorrectly reported that Ireland decriminalized homosexuality in 1933. If Ireland approves a referendum legalizing same-sex marriage, it will be the first country in the world to do so by a popular vote. (Griff Witte and Nicki DeMarco/The Washington Post)

Catholic and deeply conservative, Ireland was long known as one of the toughest places in the Western world to be gay. Homosexuality was decriminalized here only in 1993, after years of pressure from European authorities.

But now Ireland may be preparing for its coming-out party, with a referendum on Friday that could make it the world’s first country to approve same-sex marriage in a popular vote.

That such a momentous event in the gay rights struggle could happen here, of all places, reflects the breathtaking social change that has swept Ireland in recent years — and the weakening hold of the scandal-scarred Catholic Church.

The church has come down firmly against the referendum. But in a country where priests once held unquestioned sway and where 85 percent of the nation still identifies as Catholic, a large majority of Ireland appears ready to defy church teachings and vote to give same-sex partners the same right to marry as heterosexual couples.

“It’s a different era,” said Pat Carey, a former government minister who came out as gay in February, at age 67, and is campaigning for a yes vote. “There’s a whole new demographic out there that has a vision of an Ireland that’s kinder, more inclusive and more tolerant.”

The change to Ireland’s constitution could reverberate well beyond this island nation’s borders as other countries, the United States among them, are wrestling with the issue in legislation and in the courts.

Unlike in the United States, where nine Supreme Court justices will soon give their ruling, Ireland has placed the choice in the hands of its 4.5 million people — leading to a passionate and colorful campaign that has made a once-taboo subject the focus of a national debate.

Supporters say a yes vote could inspire popular movements in other countries where same-sex rights had once seemed inconceivable.

“It will show that if this society can change in that way — so quickly, so radically — then other places, places that seem very conservative at the moment, that they can also change,” said Colm Toibin, one of Ireland’s foremost writers, who left the country as a young man in part because of rampant homophobia. “It would be an example to the world.”

But to referendum opponents, a yes vote would be a deeply unsettling symbol of a society transformed beyond recognition. Abortion is still prohibited in Ireland. But same-sex marriage is seen by traditionalists as perhaps the ultimate concession to cultural relativism in a country where divorce was illegal and the sale of condoms was tightly regulated until the mid-1990s.

“We’re no longer Catholic ­Ireland,” said Evana Boyle, an organizer of Mothers and Fathers Matter, a group campaigning for a no vote. “We’re changing the ­essence of an institution that has been known as one man and one woman since the beginning of time.”

Boyle’s group has plastered this city, and much of the country, with posters showing opposite-sex parents kissing a cherub-faced baby along with the words “Don’t deny a child the right to a mother & a father. Vote No.”

Boyle, a lawyer and a mother of four, said her side is counting on a backlash to a new era in which homosexuality has become “normalized.” When even Catholic schools plan lessons around LGBT Awareness Week, she said, she needs to be on guard against attempts to indoctrinate her own children. “The idea of having two dads, they just go, ‘Eww, that’s not right,’ ” she said.

But the no side’s message that defeat would be beneficial for kids is undermined by the near-unanimity of child welfare organizations in supporting the referendum’s passage. Beyond the Catholic Church, there is little opposition to the measure within the Irish establishment.

The government backs a yes vote, as do all the significant political parties, the major media organizations, and unions and business groups. The yes campaign has also won support from sports stars and even dissident clerics. In January, a Dublin priest announced during a Saturday night Mass that he supported the referendum and that he is gay. His congregants gave him a standing ovation.

The church hierarchy has been notably quiet — releasing letters and preaching sermons against the measure but hardly putting its full weight behind the referendum’s defeat. In a country where the wounds of the church’s child sex abuse scandal are still raw, it’s unclear whether Catholic leaders could influence voters even if they tried.

“They’ve been a bit more circumspect — and they have to be, because people are still sore with them for all that they’ve done,” said David Farrell, chairman of the politics department at University College Dublin.

The days of Ireland dancing to the church’s tune, Farrell said, are gone. But that doesn’t mean a yes vote is assured.

Polls have shown that same-sex marriage is favored by a significant majority of Irish voters. But the contest is expected to tighten as the campaign enters its final days.

Farrell said the campaign for same-sex marriage risks falling victim to complacency, especially if younger voters assume that the referendum is destined to pass, oblivious to the discomfort that older, rural voters feel toward a change that many see as radical. “It’s all going to hang on who gets their people out,” he said.

And in that regard, same-sex marriage advocates have been far more visible. Rainbow flags sway in the breeze along this city’s picturesque riverfront, and a stylized photo of two men locked in a loving embrace graces many storefronts.

At the Dublin city center offices of BeLonG To, a support organization for gay and lesbian youths, a group of parents recently gathered to speak out for same-sex marriage on behalf of their ­children.

“A yes vote would mean that we can celebrate diversity and ­acknowledge that families come in all shapes and sizes,” said ­Gerard Roe, father of a 13-year-old who Roe said is still figuring out his sexual identity. “It would mean that my son could come to me one day, tell me who he loves, and regardless of what sex that person is, it wouldn’t matter.”

A few miles away, in a hardscrabble suburb, a brightly painted campaign bus pulled into a community center parking lot and was met with a rapturous welcome from dozens of residents. Over sandwiches and sweets emblazoned with the word “YES,” longtime gay rights crusaders implored the crowd to vote. A self-identified great-grandmother won deafening applause with a cry of “Live and let live!”

Carey, the former government minister who came out as gay in February, said the passion and energy around the referendum campaign were unlike anything he had seen in Irish politics.

Carey, who is bald, erudite and speaks with the soft and deliberate cadence of a man who thinks carefully about his words, taught in a Catholic school for 30 years and served in Ireland’s Parliament for 14 before fully acknowledging even to himself the truth of his sexuality.

For much of his life, he said, “being gay was not okay.” The choice was to stay in the closet, to come out and endure homophobic abuse or to leave Ireland altogether. But after decades of loneliness, he is now in a committed relationship with a man he loves and at the forefront of a national campaign that would allow them a right both extraordinary and banal: to marry, if they choose.

“We’ve traveled an amazing journey in a very short space of years,” Carey said. “To be honest, I didn’t think I’d see that in my lifetime.”

Karla Adam in London contributed to this report.

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