The campaign to secure special designated status for Northern Ireland post-Brexit is building momentum across Europe, Sinn Fein has insisted.
The republican party's Stormont leader Michelle O'Neill dismissed the notion that such a concession would be unachievable in the forthcoming negotiations.
While she described the triggering of Article 50 as a "disaster" for the island of Ireland, the Democratic Unionists hailed the start of the Brexit process as an "historic day".
The majority of voters in Northern Ireland backed Remain - 56% to 44%.
"Brexit would be a disaster for Ireland, socially, politically and economically," said Mrs O'Neill.
"It is unacceptable that Tories, who have no mandate in Ireland, can impose Brexit and a border against our will.
"The people of the north voted against Brexit in a democratic poll."
She added: "Clearly we need special status - we are building momentum and that argument is resonating across Europe."
Mrs O'Neill said the Irish government needed to "step up" and ensure special status was secured in the Brexit negotiations.
DUP deputy leader Nigel Dodds struck a very different tone during Prime Minister's Questions in the House of Commons.
"The Prime Minister has rightly been emphasising her determination to deliver for all constituent parts of the United Kingdom on this historic day," he said.
"And whilst others are content to moan and whine we want to see that delivery happen and we are confident she will make that happen."
The Ulster Unionists campaigned for Remain but, since the result, the party has voiced support for the referendum result to be actioned.
UUP MP Danny Kinahan told the Commons: "May I congratulate the Prime Minister and the Government on today triggering Article 50.
"I know this a momentous action for the whole of the United Kingdom.
"And while I, like herself, campaigned to stay in, we recognise that the people have spoken and we offer, the Ulster Unionist Party, full support in ensuring that negotiations deliver the best for the whole of the United Kingdom and particularly for Northern Ireland."