Sinn Fein leader Mary Lou McDonald warned Thursday that Brexit was "a clear and imminent threat" to the island of Ireland and was incompatible with Northern Ireland's two decades-old peace accord.
"The reality is that Brexit and the Good Friday Agreement are not compatible," the Irish republican leader said, referencing the landmark 1998 accord which devolved government to the British province and ended several decades of deadly violence.
"Brexit, whether it is hard or soft, represents a clear and imminent threat to the economic, social and the political functioning of Ireland in its totality," she added.
Northern Ireland has been without a devolved government for more than a year after Sinn Fein and the Democratic Unionist Party (DUP) pulled out of a joint executive amid political disagreements.
Months of talks, including with British Prime Minister Theresa May this week, have so far failed to bridge their differences.
McDonald said those now coming out against the Good Friday Agreement were pursuing "their own interests" of a hard Brexit at all costs.
"That's a deeply shocking and, in our view, deeply irresponsible position for anybody to take," she said.
McDonald warned the re-introduction of any kind of border on the Irish island must be avoided.
"There cannot be a border on the island of Ireland," she said.
"To impose such a measure would be catastrophic in terms of our commerce, in terms of our access to services... in terms of how people live their day to day lives."
McDonald said Northern Ireland instead should receive special designation within any future relationship agreement Britain strikes with the EU -- something May has repeatedly ruled out.