Gerry Adams has said a referendum on Irish unity is now inevitable after a dramatic shift among Northern Ireland voters.
While the DUP hailed the General Election outcome as having bolstered the union - taking 10 of the 18 seats - the Sinn Fein leader offered a different interpretation.
As the middle-ground parties collapsed, Mr Adams said unionists had only secured less than half the electorate's backing for the first time in the region's history, while nationalism had clearly turned its back on Westminster.
"One thing we can say for certainty, there is going to be a referendum on Irish unity," he said.
"I can't say when, but there is going to be."
Sinn Fein secured seven of the region's 18 seats - up three - standing on a platform of abstention from Westminster.
The more moderate nationalist SDLP lost all three of its seats in a humiliating defeat which has sparked, perhaps prematurely, predictions of an existential crisis in its ranks.
No loss wounded the SDLP more than in Foyle, the party's cradle and home of founder and Nobel Peace Laureate John Hume, a revered figure in Derry.
Elisha McCallion's shock snatch of the seat, with a razor-thin margin of 169 seats, is being celebrated as a seismic, game-changing advance for Sinn Fein.
Republican celebrations in the city continued through the night until an afternoon rally at Free Derry corner, an iconic nationalist landmark.
Dressed in sunglasses, sandals and a denim jacket, Ms McCallion, 35, told hundreds of cheering supporters that a "new era of Irish politics" was being ushered in.
Back in Belfast, Mr Adams told a press conference there was a need for calm reflection on charting a way forward.
A key issue in the coming days will be the talks to restore to the devolved power-sharing executive at Stormont, which collapsed amid a growing schism between Sinn Fein and the DUP.
Prospects for a resolution look further away than ever after the Westminster result.
While Sinn Fein insists an agreement is "do-able", the DUP comes back to the table with its hand strengthened ten-fold since doing a deal to prop up the Conservative government.
Despite a bruising performance at the last Assembly elections, the Democratic Unionists took a record 10 seats, up two, in a barnstorming result that hands it the balance of power in London.
Their chief unionist rival, the UUP, went the same way as the SDLP, losing both its two seats.
That DUP sway will mean a Tory secretary of state would unlikely be acceptable to Sinn Fein as a broker in the negotiations, due to start next week.
Nonetheless, DUP leader Arlene Foster insists she wants devolution back up and running as quickly as possible.
"(Sinn Fein) pulled it down and now they have come back with a series of red lines, so the question about devolution is really one for Sinn Fein, because we want devolution back and running," she said.
"We believe it is the best form of government for Northern Ireland and we believe in terms of Brexit that we need a distinct Northern Ireland voice and that can only be gained if we have devolution up and running again."
Mr Adams said he hoped more measured language about Sinn Fein from the DUP in recent weeks indicated "a more mature, less juvenile approach".