“Today represents a very significant moment of change. It is a defining moment in our politics and for our people,” Sinn Féin Northern Ireland chief Michelle O’Neill said.
Now, he said, there should be an “honest debate” around the party’s goal of unifying the territory with the Republic of Ireland.
The victory will not change the status of the region, as the referendum needed to leave the UK is at the discretion of the British government and is likely to be years away.
But the symbolic importance is enormous, as it ends a century of dominance by pro-British parties, supported predominantly by the region’s Protestant population.
Scottish First Minister Nicola Sturgeon, who is also leading a campaign to secede from Britain, was one of the first to congratulate Sinn Féin in a Twitter message, hailing the “truly historic result”.