The Irish electorate was tipped to reject mainstream parties in an election expected to have an indecisive result.
On Friday, polling stations countrywide were reporting a "steady" turnout in the afternoon with an average of 30 per cent.
"This one's for the future," said outgoing Prime Minister Enda Kenny as he cast his vote in his home constituency of Castlebar in western Ireland's County Mayo.
"Your friends and family need to know that the choice today is a critical one," Kenny said in a later tweet to voters.
Opposition Fianna Fail leader Micheál Martin was also out early, casting his vote in his native Cork in the south of the country.
He said he was "hopeful" that Fianna Fail would get "a good result," adding that he had met many people on the campaign trail Thursday who had not yet made up their minds.
Polling stations opened at 0700 GMT and are to close at 2200 GMT in 40 constituencies across the country.
Voters in outlying islands cast their votes on Thursday to ensure the ballot papers could reach the mainland for the count, which starts Saturday at 0900 GMT.
Fianna Fail is expected to get 20 per cent of the vote and nationalist party Sinn Fein is set for its best election performance south of the border with about 15 per cent.
Fine Gael, which polls predict will win 30 per cent of the vote, and Labour, at just 7 per cent, have ruled out a coalition with either Fianna Fail or Sinn Fein.
If Kenny sticks to his guns, independents and smaller parties could play a major role in forming the next government.
If Fine Gael does slightly better than predicted, a coalition with Labour could still be possible. But Labour would need to get at least 15 seats to enter government, which may not happen if the polls prove correct.
Some 3.1 million people are eligible to elect 157 members to the 32nd Dail, or lower house of parliament.
Under Ireland's electoral system of proportional representation with a single transferable vote, counts can take a long time, and final official results may not be available until late Saturday.
However, early unofficial tallies carried out by the parties, which tend to be very precise, will be available within a few hours of the count starting Saturday morning, and a clear picture of the trends should emerge by Saturday afternoon.