Ireland is determined to hold firm on the demand for matching customs rules on both sides of the border after Brexit, the Taoiseach has said.
Leo Varadkar said the proposal to avoid a hard border was backed by all members of the EU27, but he made clear that even if there was a softening of stance among other European leaders he would not back down.
"We have the absolute support of the other European Union member states that are remaining," he told the Dail.
"We have not come under any pressure as yet to soften our position.
"However, I am not so naive as to think that that may not occur. We will avoid at any cost being isolated. However, even if we are isolated, we have to hold to this position, in my view."
Responding to claims that the Irish and UK governments' relationship has markedly deteriorated due to their opposing views on how to manage the border, Mr Varadkar insisted personal relations remained "very good".
He added: "The difficulty is not so much one of relations or relationships but rather the enormous policy gap that now exists between a United Kingdom government that wants to leave the European Union - it seems to be pursuing a hard Brexit policy by leaving the single market and customs union - and an Irish government that accepts the decision the UK people have made but wants to protect our national interests, not only those relating to trade between Britain and Ireland but our national interests in respect of Northern Ireland as well."
He predicted a "difficult few weeks and months ahead" in the Brexit talks.
"This is potentially a historic decision for us," he said.
Politicians in Dublin and Brussels have warned that if Northern Ireland operated outside the rules of the customs union and single market it could have dire consequences for the island's economy and the cross-border provisions of the 1998 Good Friday peace agreement.
Europe is calling for a solution that would see Northern Ireland continue to be in compliance with the EU's regulatory framework, to enable the retention of a free-flowing border.
Mr Varadkar told TDs: "We think it can be best achieved if the United Kingdom, either on behalf of all of the UK or on behalf of Northern Ireland, commits to regulatory equivalence, that is to say, that we will operate the same rules and regulations. Without doing that, it is almost impossible to avoid some form of hard border."
This week DUP leader Arlene Foster accused Dublin and Brussels of acting "recklessly" in regard to Northern Ireland and claimed they were trying to use the region as a bargaining chip.
This suggestion of Northern Ireland operating under different trade regulations from the rest of the UK has angered the DUP and other unionists.
Earlier Minister for Foreign Affairs Simon Coveney claimed the UK was failing to offer credible answers to the "hard questions" Ireland and the EU were asking about the fate of the border.
Mr Coveney rejected suggestions that Dublin and Brussels are seeking to exploit the uncertainty over Northern Ireland's position to strengthen the EU's hand in the negotiations.
"We are certainly not seeking to exploit anything," Mr Coveney said on a visit to Belfast.
"We are trying to protect a peace process that so many people from all backgrounds, unionist and nationalist, have worked so hard to create."
The Fine Gael minister added: "What's happening here is we are asking the hard questions and unfortunately we are not getting credible answers, which is why I think some people seem to be uncomfortable."