“This is not equality, and for many gay people, civil partnership, while it was a step in the right direction, does not give the relationship they have and the commitment they have made to the person they love the same recognition as the relationship of a heterosexual couple,” wrote former Justice Minister Alan Shatter in a recent opinion article in The Irish Independent. “They feel discriminated against, regarded as second-class citizens, and they are hurt by it.”

Opponents have broadly defined the implications of the amendment, saying it could affect issues like surrogacy, adoption and education, as well as morality and family values.

“We are not against equality, but this is about changing the primary, natural, fundamental unit of society,” said Ben Conroy, a spokesman for the Iona Institute, a conservative advocacy group. “It is about obliterating the right of a child to a mother and father.”

While many states and other countries have moved to broaden equal rights for gays, resistance lingers, even in the West. France faced strong opposition and angry protests before it allowed same-sex marriage. In many places, including the United States, many of the rights questions are being decided in the courts.

In the Irish Republic, opponents of the measure have been careful to distance themselves from events in Northern Ireland, where religiously conservative Protestant politicians have blocked proposals to allow same-sex marriage that would put it in line with the rest of Britain.

However, the courts have stepped in there as well, and on Tuesday a judge in Belfast County Court ruled that a baker was guilty of discriminating against a customer, Gareth Lee, because of his sexual orientation when the bakery refused to provide a cake inscribed with the words “Support gay marriage.” The owners of the bakery said they refused because of their “genuine deeply held religious beliefs.” Mr. Lee agreed to accept 500 pounds, or about $775, in damages.

Ireland has been reluctant to tinker with its 1937 Constitution, particularly on social issues. Nevertheless, it has taken on two previous battles over teachings that are abhorrent to the church.