LONDON — Ireland’s state-run secondary schools can no longer assume their students will receive religious instruction, the government has said, directing the schools to offer alternative classes — a striking move in a country where education has long been dominated by the Roman Catholic Church.

Irish law already states that government-run schools cannot require students to take religion classes, which have been dominated by Christian doctrine. But that law has had limited effect, as schools have routinely enrolled all students in the courses unless their parents opted out.

The schools have not usually offered alternative classes, often requiring that exempted students remain in their classrooms during religion courses that they were, in theory, not taking.

This week, the Department of Education directed state-run secondary schools to end the opt-out requirement — so that taking a religion class would be an affirmative choice, not a default — and to offer other courses that could be taken instead.

And on Wednesday, the Teachers’ Union of Ireland called for the change to apply to all secondary schools in the country, including religious ones.

About half of the country’s secondary schools are run by governmental organizations, according to the Department of Education, including most vocational schools. But despite also being government-funded, almost all primary schools in Ireland are religious, with the vast majority run by the Roman Catholic Church.

State-run schools have long offered religious instruction in keeping with community norms — which almost always means Catholic, in a country where about four-fifths of the population identifies as belonging to that denomination.

“It may have been reasonable when these schools were originally established for a school to assume that its pupil population was predominantly Catholic and to arrange religious instruction accordingly,’’ the Department of Education said in a statement announcing the new policy.

“In a changing context, the constitutional right not to attend religious instruction must be given effect through changed practices.’’

The archbishop of Dublin, Diarmuid Martin, warning that schools are gradually losing their Catholic character, has pushed for years for the church to divest itself of hundreds of schools. But he has had little success, meeting strong resistance from the religious orders that control them.

The change on religious instruction offers the latest example of Ireland’s liberalization and the waning influence of the Catholic Church over public policy and social mores. In the 1990s, Ireland decriminalized same-sex relations and legalized divorce.

It legalized same-sex marriage by referendum in 2015, with 62 percent of voters approving the change. And in May, the country will vote on a referendum to repeal the constitutional ban on abortion.

It is a measure of how much Ireland has changed that the loudest objections to the new policy on religion classes in state-run schools have come from the schools themselves.

Teachers and administrators have objected not to having less religious instruction, but to the statement by the education minister, Richard Bruton, that the government will not provide funding for alternative classes.

“Ireland has changed tremendously, and in urban areas there will be significant numbers of people choosing other subjects rather than religion,’’ said Michael Moriarty, general secretary of Education and Training Boards Ireland, which oversees most state-run schools. “We don’t have the staffing resources to put on extra classes with extra teachers.’’