The US Supreme Court has struck down the law defining marriage as being between a man and a woman.

The ruling was passed by a vote of 5-4. It means that legally married gay men and women are entitled to claim the same federal benefits that are available to opposite-sex married couples.

In a second case, a 2008 ban on gay marriage in California has been overturned with US President Barack Obama calling the couple who brought the case to congratulate them

The Defence of Marriage Act (DOMA) had denied married gay and lesbian couples in the United States the same rights and benefits that straight couples have.

"DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment" of the Constitution, the court said in its decision.

Advocacy group Marriage Equality have welcomed the ruling. Moninne Griffith, Director of Marriage Equality said that this is an historic day for many families across America.

"Today's historic win in the U.S. Supreme court is great news for so many same sex couples, their families and their children across the U.S. and would like to congratulate all our friends in the U.S. on this huge achievement" she said.

"It will also add to the momentum for marriage equality here in Ireland too. With this momentum, Marriage Equality and our supporters all over Ireland will continue to call on the Irish Government to take on board the recommendation from the Constitutional Convention in April, to provide for access to civil marriage for same-sex couples in the Constitution by calling a referendum on the issue as soon as possible" she added.

Last week the Tanaiste Eamon Gilmore said he would be in favour of a referendum being held on this issue in 2014.