Sinn Fein, the left-wing Irish nationalist party, has won the popular vote in a general election, with the one-time political wing of the Irish Republican Army (IRA) disrupting a duopoly of centre-right parties which have historically controlled the Republic.

Ballot counts on Sunday revealed that after all 39 constituencies across Ireland were tallied, Sinn Fein received 24.5 percent of the first preference vote, almost doubling its share from the last election in 2016.

It outstripped the opposition Fianna Fail party, which won 22.2 percent, as well as incumbent Prime Minister Leo Varadkar's governing Fine Gael party's 20.9 percent.

More:

Ireland operates on a single transferable vote system and Sinn Fein ran a slate of just 42 candidates for the 159 seats contested, meaning its strong performance may not result in it becoming the biggest party in Ireland's next Parliament.

But its leaders, who campaigned on issues of healthcare and housing, celebrated the surge.

"It's official (Sinn Fein) won the election - highest popular vote," tweeted Sinn Fein President Mary Lou McDonald.

Thank you. All of you who voted for Sinn Féin. Your vote counted. We are your party. We will now do everything we can to secure a government for the people. We will never leave you behind. We will never let you down. pic.twitter.com/6uCmETLTo3 — Mary Lou McDonald (@MaryLouMcDonald) February 9, 2020

An exit poll put Sinn Fein comfortably ahead with voters aged 18-24 and 25-34, with support at 32 percent in each age bracket.

"There was, in the build-up to the election, people saying this is young people who don’t remember the Troubles in Northern Ireland and I think we see that's not the case," Dawn Walsh from the University College Dublin told Al Jazeera, referring to the 30-year period of violence and political dispute in which some 3,600 people were killed before a 1998 peace deal.

"It's only the much older people who have stuck with Fianna Fail and Fine Gael."

Counting is expected to continue on Monday with analysts saying it could take two to three days before full results are known.

Ending the duopoly

Following Ireland's 2016 election, it took 70 days before a new minority coalition government was formed under Fine Gael.

Before that vote, Fine Gael and Fianna Fail, who have traditionally taken turns in power in the Republic since its 1922 foundation, ruled out a coalition with Sinn Fein.

"It seems that we have now a three-party system," said Varadkar at the counting centre for his Dublin West constituency.

"That is going to make forming a government quite difficult."

While Fine Gael minister Pascal Donohoe reiterated the party's stance, that it would not work with Sinn Fein, Fianna Fail leader Micheal Martin did not repeat 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," he said. "But what is important is that the country comes first."

Micheal Martin of Fianna Fail reacts to being elected to the 33rd Dail at the Irish General Election count [Jeff J Mitchell/Getty Images]

McDonald arrived at the main count centre in the capital to huge fanfare from supporters and was returned to her central Dublin seat on Sunday evening.

"This is changing the shape of an old Irish politics. This is not a transient thing, this is just the beginning," she told reporters.

The 50-year-old said Sinn Fein would talk to all parties about forming a government and that others should accept their responsibility to do the same.

"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 is fundamentally undemocratic," she said.

Sinn Fein's ultimate goal is reuniting British-run Northern Ireland with Ireland.

The party has set a condition for any coalition - to immediately prepare a unity referendum that it would push London to hold within five years.

Opponents of Sinn Fein say its high-spending promises and pledge to increase taxes on the wealthy would discourage foreign multinationals that employ one-in-10 Irish workers.

But some parties have praised detailed policies such as a proposed rent freeze and building more social housing to tackle the cost and availability of accommodation, the key election issue.