Cardinal Sean Brady criticised the legislation
The leader of Ireland's Catholics has criticised civil partnership legislation in the Republic of Ireland.
In a sermon, Cardinal Sean Brady said the measures would hugely change peoples' concepts of the family.
The Civil Partnership Bill, published in June, gives Irish same-sex couples recognition as partners before the law.
The director of the Gay and Lesbian Equality Network, Brian Sheehan, said there was a democratic consensus for the new laws.
"I understand the fears of people who are afraid that civil partnership will undermine marriages," he said.
"But I think you can see from Northern Ireland and the UK that it is in the common good for people to be making lifelong commitments and the state providing a framework for that."
Under the legislation, same-sex couples will be treated like married couples for tax and social welfare.
Dr Brady said marriage between a man and a woman would always remain the ideal environment in which to raise children.
He said any government which undermines such an environment could hardly be said to be promoting the common good.
Cardinal Brady also said those who refuse to officiate at a same sex wedding would be guilty of an offence and described this as "an alarming attack on the fundamental principle of freedom of religion and conscience".
He said the bill left the door open for individuals and religious organisations to be sued for "upholding their belief that marriage is an institution exclusively for men and women".
Dr John Murray from the Catholic Iona Institute, which promotes marriage, said the cardinal was right.
"He is pointing out that marriage is fundamentally different from other kinds of loving relationships," he said.