A general election surge in support for Sinn Fein, once the political wing of the Irish Republican Army, upset Ireland's traditional two-party grip on power.
Left-wing Irish nationalist party Sinn Fein on Sunday demanded to be part of the next Irish government after tallies indicated it secured the most votes in an election that leader Mary Lou McDonald described as a ballot box "revolution".
The former political wing of the Irish Republican Army, which has reinvented itself as the main left-wing party, secured 24 percent of first preference votes, almost doubling its vote from the last election four years ago, a tally by Virgin Media TV showed.
That put it narrowly ahead of the party of Prime Minister Leo Varadkar and fellow centre-right rival Fianna Fail. Varadkar, meanwhile, said a coalition with Sinn Fein is not an option.
However Sinn Fein is likely to fall behind at least one of its rivals in terms of seat numbers as it stood far fewer candidates and can aim at best to be a junior partner in a government.
Seismic shift in Irish politics
Both Fine Gael and Fianna Fail, who have between them led every government since the foundation of the state, consistently ruled out a coalition with Sinn Fein ahead of the election, citing its policies and history.
Analysts suggested that resolve may be tested in the coming days in what many described as a seismic shift in Irish politics away from the century-old, centre-right duopoly.
"This is certainly an election that is historic... this is changing the shape and the mould of Irish politics. This is just the beginning," McDonald told a media scrum after arriving at her election count to a huge ovation from party supporters.
"I do not accept the exclusion or talk of excluding our party, a party that represents now a quarter of the electorate and I think that that is fundamentally undemocratic."
Fianna Fail leader Micheal Martin, who predicted his party would win the most seats, declined to repeat his earlier refusals to consider a coalition with Sinn Fein, saying only that there were significant incompatibilities on policy.
"Our policies and our principles have not changed overnight, in 24 hours, but what is important is that the country comes first," he told reporters in Cork. He later added that Fianna Fail's issues with Sinn Fein's history had not gone away.
The first parliamentary seats under Ireland's complex single transferable vote system began to be declared from 1600 GMT. The final and potentially decisive results may not be known until Monday or even later.
Sinn Fein has moved on from the long leadership of Gerry Adams, seen by many as the face of the IRA's war against the British rule in Northern Ireland – a conflict in which some 3,600 people were killed before a 1998 peace deal.
Irish tricolour flags were flown at a Dublin count centre as Sinn Fein supporters were led in a chorus of the Irish rebel song "Come Out Ye Black and Tans" by Dessie Ellis, a re-elected lawmaker who was jailed on possession of explosives in 1981.
Early tallies showed McDonald had more than twice the number of votes needed for election in her constituency – and the Sinn Fein candidate was well ahead of Varadkar in his four-seat Dublin West constituency.
The tallies showed a reasonable result for Varadkar's Fine Gael, in power since 2011, after opinion polls a week ago showed it in third place.
But the party's focus on the fastest growing economy in the EU and success in negotiating a Brexit deal that avoided a hard border with Northern Ireland failed to capture the imagination of voters, who were far more focused on issues like health and housing, where Sinn Fein has been strong.
Just 1 percent of exit poll respondents said Britain's exit from the EU was a factor in how they voted, Ipsos MRBI found.
Another minority government?
Both Fine Gael and Fianna Fail said before the election that they would look to smaller parties to form what would likely be another minority government - requiring the support of the other from the opposition benches via a "confidence and supply deal."
"The issue really is what kind of confidence and supply agreement could be put together and are we in a position where that's a realistic option," Fine Gael minister Richard Bruton told Reuters news agency, adding he saw "no flinching at all" in his party's ruling out of Sinn Fein.