Voter turnout exceeded 60 percent at some of the polling stations as more than 3.2 million people were eligible to take part in the referendum.
In 2010, Ireland enacted civil partnership, which gave gay couples similar, but not equal rights with those legally married.
A 2011 census revealed that some 4.000 same-sex couples cohabited in Ireland.
Gay marriage is legal in some European and South American countries, as well as Canada, New Zealand and South Africa.
Same-sex marriage is also officially recognized in parts of Mexico and most states of the United States.