Former taoiseach Bertie Ahern has said at the time the Good Friday Agreement was being conceived in 1998, "none of us mentioned the fact that the UK might be out of the EU", adding "there are a number of references in the GFA where we thought we would continue to co-operate".
Mr Ahern was speaking at a conference in Dublin, which is addressing "Brexit, Ireland and the Future of Europe".
He said the successes of the Good Friday Agreement were peace in the North, a reform of policing, decommissioning, equality legislation, and the release of prisoners.
With regard to the most recent breakdown of talks in Northern Ireland, Mr Ahern said the "problem for negotiations now is that there are a lot of new people. Some don’t fully engage with the history of cooperation, there is a different British government, different personalities have made life more complex".
On the issue of the border, Mr Ahern said the "idea of us going back to some kind of border is a disaster, communities on both sided of the border would pull down anything with their bare hands".
He said the Government here has ruled out a technical solution and "rightly so".
He added it does not seem likely the UK "will sign up for Customs Union as we know it".
Some GFA principles ‘as central as Magna Carta’
In his address, another former taoiseach, Brian Cowen, said there are some "very complex constitutional principles" from the Good Friday Agreement that are "as central as Magna Carta".
But, Mr Cowen said, some are now treating the agreement as an "expedient that can be thrown aside for leaving the EU.
He added there are citizen rights in the agreement and these are not issues that "can be dismissed lightly".
Ireland and the UK’s continued membership of the EU was part of the dynamic of an evolving relationship, according to Mr Cowen and he said "one would be very concerned at how that relationship would evolve if the UK has left".
On the border issue, Mr Cowen said "some arrangement" has to be sorted out because the "whole rural economy is based on that interdependence and it being able to continue on".
He said: "constructive ambiguity can get you so far, as we have seen with the Good Friday Agreement, but there comes a point where the concrete specifics have to be addressed".
On the UK’s Brexit Secretary David Davis, Mr Ahern said he does not know him but "when he accuses our Taoiseach of being in the pocket of Sinn Féin, he doesn’t get it too well".
‘They are a clever bunch’
Mr Ahern said he has sat across the table from the British "with all sorts of agreements - they are a clever bunch, and I have no doubt they have worked out what they can give on regulatory alignment.
"If that can bring us a solution in which there is no change of substance with the existing Customs Union then let’s buy it."
He said there is "no point in beating the drum on the Customs Union and the Single Market.
"Let’s make regulatory alignment as agreed in September the thing to nail down. It seems to me that’s where the game is.
"It’s been a bad two years since the vote, have to push forward," Mr Ahern added.
Former taoiseach John Bruton highlighted the positive results of both Ireland and the UK being in the European Union.
"Between 1922 and 1973 no British prime minister ever felt it was worthwhile going to Ireland to meet a counterpart. That indicates to me an unhealthily, unequal relationship and patronising attitude.
"That all changed when the British joined the EU. Heath met Cosgrave within months of joining the EU at Baldonnel."
Mr Bruton added that working together to solve a European issue created solidarity.