Ireland and the UK's objectives in the Brexit negotiations are not mutually exclusive, the Taoiseach has insisted.

Leo Varadkar acknowledged relations with London had become "strained" amid the wrangle over how to manage the Irish border, but said it was time for "grown up" politics as both governments seek to achieve what is best for their citizens as the EU/UK talks move on to the next phase.

The Taoiseach said he was determined to get the best outcome for the Irish people from Brexit negotiations, which he said had so far "challenged" the country's relationship with British Prime Minister Theresa May's government.

"Relations obviously are a little bit strained, and they've been challenged by the events of recent months," he said.

"The reason that relations have become strained is because of Brexit.

"Brexit was not our policy. Brexit was a decision of the UK people, which we respect, and is being pursued by the government in the UK.

"What has strained relations is that decision. But we need to be grown up about it. And we need to get on with it and try to get the best outcome for the Irish people."

Although strained, Mr Varadkar said his relationship with Mrs May's administration was "not bad".

He said he would speak with the Prime Minister every two weeks.

Mr Varadkar said he believed that he and Mrs May have a "shared and common objective", which is "to get the best outcome for our people - for her the best outcome for the United Kingdom, for me the best outcome for Irish people, both here in this state and in Northern Ireland.

"And I don't think things are mutually exclusive. In a lot of areas we'd be very similar. For example on free trade - both of us will want to have as close as possible a relationship when it comes to trade for example, so you know, I think there's a lot of work we could do together there."

Mr Varadkar and Mrs May are due to meet in January to discuss the ongoing political crisis in Northern Ireland, which has been left without a functioning government for almost a year following the collapse of the power-sharing executive.

"We're going to have another push in the new year to get the executive up and running," said Mr Varadkar.

"Very conscious that there are a number of parties in Northern Ireland, all of which represent people there.

"And I'm certain the Tánaiste (Simon Coveney) and I will be meeting them over the course of January and doing anything we can do to get those institutions up and running again."

During a pre-Christmas media briefing, Mr Varadkar also highlighted the achievement of former taoiseach, Enda Kenny, in attaining agreement among European leaders that Northern Ireland would automatically rejoin the EU in the event of Irish reunification.

He said: "The Kenny text, which I think people may have forgotten about, but was there in the early part of the negotiations, it was an assurance from our European partners that in the event that a United Ireland were ever to happen, the six counties of Northern Ireland could join the European Union in the way that East Germany did."