Ireland votes today on whether to allow gay marriage in a referendum that could make the once deeply Catholic country the first in the world to adopt the policy by popular vote.

The reform is backed by all political parties, championed by big employers and endorsed by celebrities.

Opponents, including church leaders, have raised concerns over parenthood and surrogacy rights for gay couples. But polls indicate that the referendum will be passed by a margin of as much as two-to-one.

"My message to people is that if you believe in equality, do not be complacent, do not leave it to others," said Prime Minister Enda Kenny, a practising Catholic who has spoken of his personal journey to become a leading advocate for gay marriage.

"Say yes, yes to inclusion, yes to rights, yes to love, yes to equality. Take away those burdens for people and let them be who they are."

A number of US states recognise gay unions as do Brazil, France, New Zealand, Britain and other countries, but none have extended civil marriage to same-sex couples by way of a national referendum. Recent votes in Slovenia and Croatia were rejected.

Pollsters say a ratification may hinge on whether younger voters, tens of thousands of whom have registered to vote in recent months, actually turn up to cast their ballot.

Results, which will be declared on Saturday, will also be closely watched for an urban/rural split. When voters legalised divorce by a razor thin majority in 1995, only five of the 30 constituencies outside Dublin backed the proposal.

Shorn of much of its influence after a series of sex abuse scandals, the Catholic Church has mainly limited its campaigning to sermons to its remaining flock but nevertheless made a last minute appeal to voters.

"I think the days when Bishops tell people how to vote is long since gone but we have constantly said this is not a simple thing," Archbishop of Dublin Diarmuid Martin told national broadcaster RTE in a rare interview.

"Marriage isn't just about two people falling in love, marriage and family and children are all linked together and you can't tear them apart."