Party leaders have urged the people to make their vote count as polls opened in the most eagerly-anticipated election in decades.
Almost 3.2 million voters are registered to cast their ballots with 550-plus candidates running in 43 constituencies for 165 of the 166 seats.
Ceann Comhairle Seamus Kirk is automatically returned.
Polling stations opened at 7am and party leaders have been voting in their constituencies.
In Castlebar, Co Mayo, Taoiseach-in-waiting Enda Kenny was accompanied by his wife Fionnuala and daughter Aoibhinn, a first-time voter, as he urged people to cast their ballot.
"I hope that as many people as possible, all over the country, go and cast their vote today," he said.
"The more people who vote, the stronger the message within our democratic system is.
"So between here and 10pm tonight, I would ask people all over the country to be sure and go out and cast their vote.
"That's the essence and the strength of our democratic system."
Fianna Fail leader Micheal Martin, accompanied by his wife Mary in Cork, said it would be a competitive election in every constituency.
"Every vote counts and I would urge people to come out and vote," he said.
Labour leader Eamon Gilmore voted in his Dun Laoghaire constituency with his daughter Grainne.
Opinion polls have put Fine Gael well in the lead to head the next government, securing as much as 40pc of the popular vote - potentially allowing for a single-party government propped up by independents.
But while the main Opposition party is firmly on track to replace Fianna Fail in power, the Labour Party warned voters against giving a party a "monopoly of power".
While backing for Labour has flagged during the campaign, Mr Gilmore has appealed for support to ensure a coalition government.
Voting on islands off the west coast took place over the past two days, with the Aran Islands off Galway taking to the polls yesterday. They saw a turnout around mid 70pc.
Parties also used the final day of a near-four week campaign, dominated by the economic and jobs crisis, to make frantic last-ditch appeals for support.
Mr Gilmore urged voters to come out and make their views known and Mr Martin attacked Fine Gael's five-point plan, the central plank of its election manifesto, claiming it was neither credible nor costed.
Despite Mr Martin's popularity after taking the helm of Fianna Fail from outgoing Taoiseach Brian Cowen, the party, which has been in power for 60 of the last 80 years, languishes at record lows in the polls.
Sinn Fein, which stands to gain seats for its outspoken opposition to government cuts and the controversial bailout from the International Monetary Fund and Europe, claimed the party would put "real political backbone" into the Dail.
Party president Gerry Adams is standing in the border county of Louth for the first time after he resigned as a UK MP.
Meanwhile, the Greens, the outgoing junior coalition party, claim they are fighting for the last seat in five Dublin constituencies and in Louth, Carlow-Kilkenny, Galway West and Cork South Central.
Elsewhere, a record number of independents are also hoping to secure a seat.
We have had many crucial general elections but this ranks as the most important, certainly in living memory. The first general election, held in 1923 just after the bloody catastrophe of the Civil War, helped give us a stable form of parliamentary democracy, which served the country well for many decades.