Former Irish taoiseach Bertie Ahern has questioned whether David Davis understands the Brexit issues.
Mr Ahern said the agreement in phase one of negotiations that there would be no hard border between Ireland and Britain after the UK's split from Europe was a fudge and not credible.
He said the debate over how trade and travel will operate between Northern Ireland and the Republic and across the Irish Sea will have to be revisited.
After attending a Brexit conference in Dublin, Mr Ahern offered a harsh critique of the Brexit Secretary.
"David Davis is a very interesting character but I watched him yesterday ... I continue to have my doubts - does he understand this stuff?" he said.
Mr Ahern said he presumed UK officials have worked out what they want in future relations with Europe.
"But the question comes back all the time - how can they get these things and be out of the single market and out of the customs union?" he said.
Mr Ahern said the agreement hammered out before Christmas on the Irish border would not survive legal tests.
"It was a fudge," he said.
Mr Ahern said international media have been describing the deal as provisional, whereas in Ireland it was being treated as a final deal.
"If you look around the world that's where it is coming from. The British are pumping that out. I think we are going to have to rerun all of that again," he said.
"It's one thing fudging a statement or communique, which I've done many times in Europe, but that now has to be put into a legal context. You cannot fudge a legal context."
Mr Ahern was in the audience at the conference in Dublin City University's new Brexit Institute, with speakers including Hilary Benn, chairman of the UK's House of Commons Committee on Exiting the EU.
Mr Benn he could not see another referendum on Brexit issues.
"David Davis was talking yesterday about the border between Canada and America and gave the impression it was all very quick and easy," he said.
"But there is infrastructure, there are queues, people have to show some things."