The warning is contained in a submission by the Irish Catholic bishops to the Constitutional Convention, which is considering the issue of same sex marriage.
The bishops’ stance would affect the thousands of weddings that take place in churches every year.
Their submission is one of over 1,000 on the issue.
Glen, the gay and lesbian equality network, estimates the submissions are about 3:1 in favour of extending access to civil marriage to lesbian and gay couples.
In their 10-page submission, the bishops say any change to the definition of marriage would mean the Church could no longer co-operate with the civil aspect of marriage. They state this would affect 70% of marriages where both the Church and civil elements take place together.
Point 29 of their submission states: “It is important to note that in Ireland, the Church and State co-operate closely in the solemnisation of marriages.
“Any change to the definition of marriage would create great difficulties and in the light of this, if there were two totally different definitions of marriage, the Church could no longer carry out the civil element.”
For a wedding to be legally recognised, it must be solemnised by a person on the register of solemnisers. About 4,300 of the 5,600 people on the register are Catholic priests.
A spokesperson from the Catholic Communications Office said: “At the moment on behalf of the State, the priest acts as the solemniser of the marriage between a woman and a man. Obviously if the definition of marriage changes then this role will change.”
Figures from the Catholic organisation Accord showed 14,232 people attended marriage preparation courses last year. According to the CSO, there were 19,879 marriages in 2011.
A Glen spokesperson said they respected the freedom to practise religion and were not seeking to force religious solemnisers to carry out same-sex marriages.
Elsewhere in the submission the bishops say marriage is not merely a private institution, but “a social institution has evolved mainly for the benefit of children”.
They also point to “a substantial body of research” which they say shows “the best outcomes for a child are most likely to be found where a child has two parents, a father and a mother, who are bound to each other in a stable marriage”.
In response, the spokesman for Glen said: “Our laws must protect and cherish all children equally, in whatever family form that they are born into, nurtured and raised within.”
The bishops conclude that same-sex marriages reduce marriage to an “arrangement of the sexual relationship of any two people” and marriage “would cease to be the institution upon which the family, and therefore society itself is founded”.