Former Taoiseach John Bruton has challenged the Democratic Unionist Party and the UK Government to spell out exactly the sort of Brexit they want.
Mr Bruton said that neither had "come forward with their own ideas" on issues such as the Irish border or remaining in the customs union after the UK leaves the European Union.
He added that the English overruled the wishes of Northern Ireland and Scotland and they now need to "change their opinion".
Relations between Arlene Foster's party and the Irish government remain strained after Taoiseach Leo Varadkar said Ireland would not help Britain design an economic border for Brexiteers.
He said Brexiteers were the ones who wanted a border, so it is up to them to design one.
Mrs Foster described the comments as "not helpful".
Speaking on RTE's Good Morning Ireland on Thursday, Mr Bruton said while it looked likely that Brexit would now happen, "we have to do everything we can to stop it happening, if we can".
"Really, it is the English. They have to change their opinion. They have initiated Brexit. The opinion in Northern Ireland was against Brexit, and the opinion in Scotland was against Brexit. That has been overruled," he said.
Mr Bruton added that the DUP should be more up-front with its own Brexit ideas.
"Some hard questions have to be asked. What sort of Brexit do they want? Do they want the UK in the customs union? What sort of agricultural policy do they want, because that will be very important at the border?" he asked.
Mr Varadkar is due to meet with Stormont's political parties on Friday on his first visit to the region since becoming Taoiseach.
He also intends to take part in a gay pride event in Belfast on Saturday morning.