Taoiseach Leo Varadkar has said Boris Johnson's Brexit plans "fall short in a number of aspects".
On a visit to Sweden, 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.
On the issue of the proposed Stormont veto on new regulatory arrangements, he said no one party should be able to block the majority view on the island.
"Any consent mechanism and democracy mechanism must reflect the views of the majority of people in Ireland and Northern Ireland," said the Taoiseach.
Mr Varadkar said he welcomed 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 British Prime Minister's customs proposals.
"We need to explore in much more detail the customs proposals that are being put forward as it's very much the view of the Irish government and the people of Ireland, north and south, that there shouldn't be customs check points or tariffs between north and south," he said.
On the petition of concern voting mechanism at Stormont, Mr Varadkar said: "Our view is that any consent mechanism, where it to exist, would have to be reflective of the view of the whole of population of Northern Ireland and not give any one party or any domination a veto."
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 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.
"What I would say is we need to be practical about any arrangements when it comes to consent or democracy clauses.
"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?
Mr Varadkar said the new economic investment package for Northern Ireland proposed by Mr Johnson as part of his plan was a positive development.
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 and customs union; the border backstop mechanism; or the UK reversing the Brexit decision.
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."