The Irish premier has insisted a Brexit deal cannot hand the DUP a veto over the wishes of the majority in Northern Ireland.
Taoiseach Leo Varadkar said no one political party on the island should be able to wield that power on the shape of post-Brexit cross-border arrangements.
Mr Varadkar, on a visit to Sweden, said Boris Johnson’s Brexit plans “fall short in a number of aspects”.
He questioned how the Prime Minister could reconcile his pledge for no new customs infrastructure on the island, with his wish to take Northern Ireland out of the customs union.
Mr Johnson wants to give the currently defunct Northern Ireland Assembly a vote, both on whether to opt into a proposed all-island regulatory system, and whether to remain in it after an initial four-year period.
A contentious Stormont voting mechanism, called the petition of concern, means a bloc of Assembly Members from either the nationalist or the unionist community can veto certain decisions, even if a majority of MLAs back them.
Mr Varadkar said the mechanism could not be a factor in Brexit arrangements.
“Our view is that any consent mechanism, were it to exist, would have to be reflective of the view of the whole of the population of Northern Ireland and not give any one party or any denomination a veto,” he said.
“Any consent mechanism and democracy mechanism must reflect the views of the majority of people in Ireland and Northern Ireland.
“No one party – not my party, not Sinn Fein, not the DUP – should be in a position to veto what would be the will of the majority in Northern Ireland or Ireland.”
The Taoiseach noted that Stormont had not sat for almost three years, due to a political dispute between the DUP and Sinn Fein.
“What I would say is, we need to be practical about any arrangements when it comes to consent or democracy clauses,” he said.
“Stormont has not sat for three years, that is the reality of it, so if we wrote into an international treaty, provisions that required certain actions by Stormont, what would happen if Stormont wasn’t operating?
“You would need a fall-back position there as well. So I think that’s the kind of thing that we need to explore with our British friends.”
Mr Varadkar was asked whether a reform of the petition of concern mechanism at Stormont could pave the way to a resolution.
“I think all these things have to be discussed, but probably best not discussed in the forum of a press conference without having the other players here,” he said.
After talks with Swedish prime minister Stefan Lofven in Stockholm, Mr Varadkar said he could not fully understand how the UK envisages Northern Ireland and Ireland operating under different customs regimes, without the need for customs posts.
Mr Varadkar said he was reassured by Mr Johnson’s comments that there would be no new customs infrastructure installed, but he said that pledge appeared to contradict the text of the UK plan, which indicated checks could be carried out at unspecified designated locations.
He said there was a need to “tease out” the detail of the Prime Minister’s customs proposals.
Mr Varadkar said the operation of two different custom zones on the island would create a “real difficulty”.
“Our objective is very clear, we don’t want to see any customs posts between north and south, nor do we want to see any tariffs or restrictions on trade between north and south,” he said.
“They were all abolished in the 1990s and we don’t want to go back to that.
“The majority of the people in the north don’t, and the majority of the people in the Republic of Ireland don’t.
“But if we are going to be in two different customs unions I think that’s going to create a real difficulty, that’s going to be very hard to reconcile.”
Mr Varadkar said there were five ways to avoid a hard border – the reunification of Ireland; the Irish Republic re-joining the UK; the UK remaining in the single market or customs union; the border backstop mechanism; or the UK reversing the Brexit decision.
“There are one in five ways that this can be done, and at least four of those would be acceptable to the Irish government, but the best one is of course a backstop or some form of backstop, and that’s what we are trying to achieve,” he said.
On the prospect of the UK staying in the EU, the Taoiseach said: “All the polls since Prime Minister Johnson became prime minister suggest that’s what the British people actually want, but their political system isn’t able to give them that choice.”
Mr Varadkar said the new economic investment package for Northern Ireland proposed by Mr Johnson as part of his plan was a positive development.
His remarks in Sweden echoed the comments of deputy premier Simon Coveney in Dublin earlier on Thursday.
Mr Coveney said that, while the plan tabled by the Prime Minister did not provide the basis for a final agreement with the EU, it did represent “progress” and could potentially be a “stepping stone” towards a resolution.
Addressing the Dail parliament, Mr Coveney said: “We cannot support any proposal that essentially suggests that a minority can determine what the majority have to live with, it’s just not going to work.
“And if that is the proposal, I believe it would be very, very difficult to get an Executive up and running.
“Why would other parties buy into an Executive if they believe that Executive could essentially prevent solutions linked to Brexit?”
Mr Coveney also said the proposal for Northern Ireland to operate under a different customs regime from Ireland presented “legal and technical” problems.
The Tanaiste said he believed Mr Johnson wants a deal, but he said people have a right to be sceptical, suggesting Ireland has not been treated well by the UK to date.
“Ireland has not been treated well at different periods during these negotiations, because the policy and the proposals and the approach of the UK has shifted towards Ireland at different times,” he said.
“But, that being said, we need to work with the negotiating team that is there in front of us, that’s what we are doing.
“There is a serious proposal now on the table, it won’t be the basis of final agreement but I hope it can be a stepping stone in that direction.”