The first same-sex civil partnership is expected to be held in the Republic of Ireland within months.
A new law recognising gay couples comes into effect on 1 January, 2011.
It secures a range of rights over shared homes, maintenance payments and pensions.
As parties have to give three months' notice to registrars, the first ceremony is expected to be in April. Gay lobby groups have praised the government for the legislation.
Irish Justice Minister Dermot Ahern said the new law reflected the many different relationships in modern Irish society.
"Gay couples, whose relationships have not previously been given legal recognition by the State, may now formalise their relationships in the eyes of the law and society at large," he said.
"Their relationships will be legally recognised and protected.
"Persons in committed gay relationships who wish to share duties and responsibilities now have the choice to register their partnership and become part of a legal regime that fully protects them in the course of that partnership and, if necessary, on its termination."
The Civil Partnership and Certain Rights and Obligations of Cohabitants Act 2010 was enacted last July, but its commencement order was signed on Thursday by Mr Ahern.
The Irish Department of Justice said that although couples must give three months' notice to registrars, a court order to waive the notice period for exceptional reasons, such as in the event of serious illness, can be obtained.
Following registration of a civil partnership, the couple will be treated in the same way as spouses under tax and social welfare codes.
The changes required to introduce civil partnership into the tax and social welfare codes also come into effect from 1 January.
Mr Ahern said the act also established a redress scheme for unmarried opposite-sex couples and unregistered same-sex couples to protect vulnerable co-habitants.
The Gay and Lesbian Equality Network (Glen) paid tribute to Mr Ahern for his efforts in establishing the legislation.