People warned to mark ballot paper with an X - and nothing else
Voters have been warned not to take 'selfies' when they exercise their democratic right today.
Around 3.2 million people are entitled to vote on marriage equality and on lowering the age of qualification to stand for election as president from 35 to 21.
Constituents in Carlow-Kilkenny will also be asked to choose a new TD in a Dáil by-election.
After weeks of keen public debate, those on both sides of the marriage referendum debate yesterday stood back and said it was now up to the Irish people.
Same-sex marriage is available in a score of countries world-wide but Ireland is staging the first referendum on the issue and today's vote has attracted global media interest.
Polling stations open at 7am and will close at 10pm. The count begins in the 43 constituencies at 9am tomorrow.
Tallies could give a good indication of the outcome by mid-morning on Saturday, depending on how close the result is. The chairman of the Independent Referendum Commission, Mr Justice Kevin Cross, has urged everyone eligible to vote to exercise their franchise.
Voters will be presented with a white ballot paper for the marriage vote and a green one for the presidential age referendum.
The voters are advised to simply put an X in the Yes or No slot and write nothing else on the ballot paper.
Judge Cross also strongly advised people to respect the law and not take 'selfie' photographs inside the polling station as this is a serious breach of the law.
Officials said voters had in most cases been issued with polling cards through the post but this is not required to vote.
Voters are advised to bring some form of identification with them to the polling station.
The Government parties have put huge resources into the campaign.
But those advocating a No argued that there was a late swing in support.
Advocates of a Yes are hoping for a bigger than average turnout today. They took the late surge in registration of more than 60,000 voters who were added to the electoral register in recent weeks and the engagement of young campaigners as hopeful sign in this regard.
Canvassing wound down last night, but at that stage voters on the islands off Cork, Donegal, Mayo and Galway had already gone to the polls. This maintains a tradition of early offshore polling days for fear bad weather would disrupt the process.