A hard border dividing Ireland is inevitable unless a special EU status is secured for the North, Gerry Adams has said.
At a major Brexit summit, the Sinn Féin leader said the Government's refusal to back widespread calls to negotiate a special status is a "grave mistake".
"Without such a designation a hard border is inevitable," he told the all-island civic dialogue forum in Dublin Castle.
The summit is the second all-island gathering hosted by Taoiseach Enda Kenny to help form Ireland's response to Britain's decision to pull out of the EU.
Mr Kenny said when Theresa May triggers Article 50 to begin the process, Ireland faces the "most important negotiations in our history as an independent state".
"We must not return to a hard border or create a new border of the future."
The Government is opposed to a special EU status for the North, warning it could set a precedent that would worry other European countries.
But Micheál Martin said the North is a special case.
"For a range of reasons which we have outlined in detail we believe that Northern Ireland is a unique case and it should have a special status," he told the gathering.
"Given just the fact that Northern Ireland will contain the largest concentration of EU citizens outside of the EU, it is different."
Mr Martin warned Brexit is the greatest threat to Ireland since the State was founded.
Under the Good Friday Agreement peace deal, people in the North have a right to either or both British and Irish citizenship, and therefore EU citizenship.
A majority of the North's voters backed remaining within the EU in last year's in/out referendum.
But the Democratic Unionists, which is snubbing the all-island forum, campaigned for a Leave vote.
Stephen Farry, deputy leader of the Alliance Party, warned the imminent hard Brexit threatens the security of the North as well as plunging it into an economic and social backwater.
"The choice really isn't between a one size fits all Brexit for the UK as a whole and special status for Northern Ireland.
"The reality is Northern Ireland is already a special case.
"The choice is having that properly managed and negotiated or having an unholy mess with Northern Ireland becoming a major anomaly."