More than 3 million people have been in invited to vote over 15 hours in Ireland’s to decide if they want the country’s constitution to allow same sex marriage.
Polling will start at 07:00 BST and will continue until 22:00 BST.
Gay marriage is currently legal in 20 countries worldwide but Ireland will be the first country deciding whether to make it legal through a referendum.
Irish voters will be asked whether they agree with the statement: "Marriage may be contracted in accordance with law by two persons without distinction as to their sex."
Homosexual acts were illegal in Ireland until recently, 1993.
The government introduced civil partnership legislation in 2010 which provided legal recognition for gay couples but not the right to marry.
The main difference between civil partnership and marriage is that marriage is protected in the constitution while civil partnership is not.
Ireland’s Prime Minister, Enda Kenny, asked voters to vote yes “for love and for equality” on a TV interview last night.
However Catholic intellectuals, writers and activists say that a yes vote will create a crisis of personal conscience in Ireland
Counting the votes is due to start on Saturday and the results will be revealed the same day.