Former taoiseach Bertie Ahern has said the return of a hard Border in Northern Ireland would be “disastrous” and offer a target to dissident republicans.
In an interview on BBC's Newsnight programme on Friday, Mr Ahern said any such move would be a "huge setback".
“I don’t think we are ever going to go back to the watchtowers or the huge security presence, I don’t think anyone is suggesting that,” he said.
“It will undermine so much of what successive governments have done.
“Of course the people who will be drinking the champagne will be dissidents because they will see this as great for them; it will give them a target again.”
Mr Ahern who helped negotiate the Good Friday Agreement said a hard Border would be “disastrous” given the amount of effort that had been made by people to avoid it.
“To consider going back to that is just unbelievable.”
Reunification referendum
He also said that while a reunification referendum was now back on the agenda due to Brexit, he did not feel it was the appropriate juncture.
“People are actively looking at what shape it would be, how would it happen,” he said.
“My own view is that there would be a time to discuss that; it’s not now because we still have the institutions not up and running. We still have too much of an unsettled climate to be having votes on it.”