The country's Catholic bishops are distributing a 16-page document to 1,300 parishes explaining their opposition to next year's same-sex marriage referendum.
They say a victory for the Yes side could effectively deprive children of the right to a mother and father.
However, the Gay and Lesbian Equality Network (GLEN) has said the "Irish Constitution should allow all loving, committed couples who wish to enter a civil marriage the opportunity to do so".
GLEN Chair Kieran Rose said "allowing lesbian and gay couples to get married will not harm or change any existing marriage, or any future marriage of a man and woman".
He added that the introduction of same-sex marriage would strengthen the place and status of marriage in society.
The referendum will be held in spring next year.
In their pastoral statement, the bishops say same-sex marriage would be a contradiction in terms, because marriage is, of its nature, a committed relationship between a man and a woman which is open to the transmission of life.
The statement, 'The Meaning of Marriage', says that in the Constitution "the State pledges itself to guard with special care the institution of Marriage on which the family is founded and to protect it against attack".
The bishops say that any attempt to alter this protection "would be a radical change in the meaning of marriage - the 'foundation stone' of society" - in the Constitution which expresses the State's foundational values.
Bishop of Elphin Kevin Doran said each episcopal conference around the world had been asked to "begin a conversation within the local Church, about the meaning of marriage" and the pastoral statement was prepared in the context of the proposed referendum.