Former Irish Taoiseach John Bruton has accused the DUP of dealing in "platitudes" and called for Ireland to push back against Brexit.
Speaking on RTE's Morning Ireland radio programme, former Taoiseach Bruton said that it was "most likely" that Brexit will go ahead, but that Ireland should "do everything we can to stop it happening if we can".
"Really the English have to change their opinion. The English are the ones that have initiated Brexit. The opinion in Northern Ireland was against Brexit, the opinion in Scotland was against Brexit, but that is being overruled by English opinion, so England has to change its mind," he said.
On the DUP part in the Brexit process, the former Fine Gael leader said that "hard questions" had to be asked of the party and its leader Arlene Foster.
"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 have an impact on the border," he said.
"The DUP, who are very influential now in Westminster particularly in the absence of Sinn Fein, they have a responsibility to answer some questions."
He added that the DUP needed to give greater clarity Britain's post-Brexit future, and that so far the party had "just talked in platitudes".
In the interview former Taoiseach Bruton also hit out at the British government for failing to put forward a clear plan for a post-Brexit border and accused them of "still having debates that they should have had three years ago".
He also noted that Sinn Fein's abstentionist policy meant that there would be no nationalist representation to make amendments to the European Union (Withdrawal) Bill when it is tabled again on September 7 after the House of Commons returns from its summer recess.
Mr Bruton's comments come ahead of Taoiseach Leo Varadkar's visit on Friday to Northern Ireland, with relations between the DUP and the Irish government growing strained in recent weeks.
The Taoiseach drew the ire of the unionist party last month after there was some suggestion from his government that it could pursue the option of a sea border between Britain and Ireland.
Ahead of her meeting with Mr Varadkar, leader of the DUP Arlene Foster said that the Taoiseach was "disrespecting" the view of the British people by saying that it was his hope that Brexit wouldn't happen.