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".
Speaking on RTÉ's Morning Ireland, 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 said.
Taoiseach Leo Varadkar is due to meet Stormont's political parties tomorrow on his first visit to Northern Ireland since becoming Taoiseach.
He said Brexiteers were the ones who wanted a border, so it is up to them to design one.
Sinn Féin President Gerry Adams welcomed the Taoiseach's remarks as he said they reflect what Sinn Féin have been saying since day one.
Speaking on RTÉ's News at One, Mr Adams said: "we don't want the border, we want rid of the border."
He said his party continues to argue numerous issues in the face of Brexit.
"In the Dáil and the Seanad we have argued for special designated status for the North in the EU. The Dáil supports that. The people of the North voted not to leave the European Union."
He said if the Tories have their way, there is no other outcome except for a hard economic border.
"What we need is to have very clear positions in terms of chasing the British down on this issue. Getting them to respect the votes of the people in the North and bringing about arrangements," he said.
He added that the best outcome of Brexit is by ensuring the other EU states see that the only viable solution is to ensure there is special designated status for Northern Ireland in the EU.