Former taoiseach Bertie Ahern has said a physical border in any shape between the two parts of Ireland in the wake of Brexit could destabilise the peace process.
Speaking to The Observer newspaper in Britain, Mr Ahern said British Prime Minister Theresa May seems to be "switching her language".
"She's saying not that there'll be no border, but that the border won't be as difficult as to create problems. I worry far more about what's going to happen with that," Mr Ahern said.
He said he was worried that would take away what he called the "calming effects" of the open border.
Mr Ahern, who was taoiseach when the Good Friday Agreement was signed in 1998, added that any physical border "psychologically feeds badly into the nationalist communities".
"People have said that this could have the same impact on the nationalist community as the seismic shock of the 1985 Anglo-Irish agreement on unionists, and I agree with that.
"For the nationalist community in Northern Ireland, the Good Friday agreement was about removing barriers, integrating across the island, working democratically in the absence of violence and intimidation - and if you take that away, as the Brexit vote does, that has a destabilising effect," said Mr Ahern.
During a visit to Dublin last month, Mrs May said both her government and the Irish Government want a "seamless, frictionless border" to continue to see trade and the continuation of the Common Travel Area.