It would herald a major breakthrough for Sinn Fein south of the border if this is reflected when the votes are counted tomorrow.
In Dublin, a stream of voters made their way to polling stations.
Surging support for Sinn Fein, which is committed to the reunification of Ireland, threatens the country's political equilibrium even though the party is unlikely to form the next government because both Fine Gael and Fianna Fail refuse to work with it.
Varadkar, the country's first openly gay leader, became Taoiseach - prime minister - in 2017 after the resignation of his predecessor.
Vardakar, however, was forced to call for elections in December when relations began to fray with FIanna Fail leader Micheal Martin and several independent lawmakers needed for a majority in the Dail Éireann - the lower house of the Irish Parliament.
Brexit did not feature prominently in a campaign that was instead dominated by domestic issues such as spiralling rental prices, record-breaking homeless numbers, controversy over the state pension age and a struggling health service.
Failure to do so could present an "existential threat" to the Irish economy, he said.
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Varadkar has warned voters that Brexit is "not done yet", as London prepares for talks with Brussels to secure a longer-term trade deal in record time before the end of this year.
However, David Farrell, a professor of politics at University College Dublin, told the Reuters news agency that the elections could be critical for Sinn Fein, which moved younger voters on the decisive issue of the elections: cost and availability of housing.
"The gains that Sinn Fein is making are consistent with what we are seeing in other democracies".
Sinn Fein's Northern Ireland Finance Minister Conor Murphy has apologised to the Quinn family over claims he made in 2007 about their son Paul who was murdered.
Sinn Fein denied Irish republicans were involved in the killing, but the party was put on the defensive. McDonald, a 50-year-old Dubliner, has helped Sinn Fein shed its hard-line image since replacing Gerry Adams, a Belfast native who led the party from 1983 to 2018.
The surge of support took Sinn Fein by surprise after it sank to 9 percent in local elections previous year.
No party is expected to reach the 80-seat threshold to enable it to govern on its own and a coalition administration of some complexion is nearly inevitable.
In the last major survey of the electorate before polling day, Sinn Fein was leading the popularity stakes on 25 per cent, with Fianna Fail second on 23 per cent and Mr Varadkar's party on 20 per cent.
"They want a change in representation and they want a change in government".
But it is thought that Sinn Fein may be unable to find enough like-minded left-leaning allies to form a workable government.