Ireland's prime minister warned Britain on Thursday that there is not much time left to conclude a smooth withdrawal from the European Union, as the head of the EU's executive arm pledged its full support for Dublin's stance.

The border between Northern Ireland and Ireland - Britain's only land frontier with the EU after Brexit - is proving the thorniest issue in Brexit talks and EU leaders will express concern on Friday over the lack of progress.

While both sides agree they must have a contingency plan to keep the border open, a so-called backstop solution was only agreed in very general terms in December and there is no consensus on how to put it in place by Brexit day in March 2019. 

In our meeting we were in full agreement that there is now an urgent need to intensify our efforts if we are to get there

"Let me be blunt, there isn't much time left if we are to conclude an agreement and to have it operational by the time the United Kingdom leaves the European Union," Leo Varadkar told a news conference after talks with European Commission President Jean-Claude Juncker and chief Brexit negotiator Michel Barnier.

"In our meeting we were in full agreement that there is now an urgent need to intensify our efforts if we are to get there."

With British Prime Minister Theresa May struggling to agree a position on Brexit within her own government, Irish Foreign Minister Simon Coveney said the focus needed to move from London to Brussels and that May had to honour her border commitments.

It was also a time for Ireland to hold its nerve, he added in an interview with the Newstalk radio station.

That position was backed by Juncker, who said the support of European Union members for Ireland's Brexit position will not change, agreeing that major steps needed to be taken by Britain in the talks and that there was not much time left.

"We wanted to make it clear again and again that Ireland is not alone, we have Ireland backed by 26 members states and by the Commission and that will not change," Juncker said ahead of an address to both houses of Ireland's parliament.

"I am strongly against any temptation to try to isolate Ireland. Ireland has to be part of the deal."