CARRIGTWOHILL, Ireland — When it comes to the Roman Catholic Church, Judy Donnelly has been something of a rebel over the years. Like much of Ireland, she supported contraception, voted in a referendum to legalize divorce and, three years ago, backed same-sex marriage.

That last vote was joyously celebrated around the country and the world, placing Ireland, which elected its first gay prime minister last year, at the vanguard of what many called a social revolution.

But when it comes to the historic decision on legalizing abortion, which will be put to the nation on Friday, Ms. Donnelly says she will vote no, as will enough of her countrymen and women, including lawmakers across the political divide, to throw the referendum result into doubt. Polls for the May 25 vote have narrowed so tightly in recent weeks that “yes” and “no” campaigners are not able to confidently predict a victory.

“It’s just not the same,” said Ms. Donnelly, 46, who works in a pub in Carrigtwohill. “It’s about values and morals.”