Speaking in the Dáil, the Taoiseach has said that the Good Friday Agreement "is not up for negotiation" in response to comments made by DUP leader Arlene Foster.

The Belfast Agreement is not up for negotiation in the Brexit talks, Taoiseach Leo Varadkar has said.

Mr Varadkar told the Dáil that the Government would stand by and defend the primacy of the agreement.

“We see the Government as being co-defenders of that agreement,” he said.

“And certainly as far as this Government is concerned the Good Friday Agreement is not up for negotiation in these talks over Brexit.”

The Taoiseach was responding to Sinn Féin leader Mary Lou McDonald who raised the issue following the publication of a newspaper interview in which DUP leader Arlene Foster said the Belfast Agreement was not sacrosanct and could be changed to facilitate a Brexit deal.

Ms Foster had expressed deep frustration with EU officials and those in favour of the UK remaining in the union, who repeatedly stated that the Belfast Agreement could not be touched.

DUP leader Arlene Foster has said the Belfast Agreement should not be considered untouchable in Brexit negotiations. Photograph: Liam McBurney/PA Wire Ms McDonald said: “The DUP rejected the Good Friday Agreement. They embrace Brexit and on both counts they act in defiance of the wishes of the people in the North.”

She said “the Good Friday Agreement is not up for renegotiation as part of Brexit and it is sacrosanct”.

She also accused the DUP of “play-acting” and playing to extremists.

Mr Varadkar said he had worked very well with Ms Foster when they both had responsibility for tourism in their respective jurisdictions.

“We should all respect the Good Friday Agreement and we should all work as hard as we can to ensure those institutions are back up and running again.”

But he said the agreement “is not a piece of British legislation. It is an international agreement between the British and Irish governments as well as a multi-party agreement among the various parties.

He said: “It could only be changed with the agreement of the British and Irish Governments and it could only be changed with the consent of the people of Northern Ireland and indeed cross-community agreement at that.”

Ms Foster, speaking to the Daily Telegraph, had said the Belfast Agreement should not be considered untouchable in Brexit negotiations.

She said: “It has been deeply frustrating to hear people who voted Remain and in Europe talk about Northern Ireland as though we can’t touch the Belfast Agreement.

“Things evolve, even in the EU context.”

Mrs Foster also said she wanted to see more focus on the positives of Brexit.

“I think the reason why so many people are turned off by Brexit is because they are being fed a diet of negativity - whether it’s infighting, Brussels, being disrespected by people over there.

“We haven’t been able to talk about the aspirations for the nation [because] we’ve spent so much time arguing about what’s happened, is it going to be a disaster for Ireland in inverted commas, instead of actually focusing on what we can achieve in the UK with the Brexit negotiations.”

Boris Johnson Mrs Foster was speaking ahead of the Tory party conference where Boris Johnson will use his speech to issue a clarion call to activists to “believe in Conservative values”.

In what will undoubtedly be seen as a pitch to replace Theresa May as leader, Mr Johnson will not only restate his opposition to the prime minister’s handling of Brexit but call on Tories to focus on law and order, tax cuts and house-building in order to defeat Labour.

As Mrs May celebrated her 62nd birthday, Mr Johnson was pictured jogging through a field near his Oxfordshire home, in a photo apparently designed to mock the prime minister’s famous memories of “running through wheatfields” as a mischievous schoolgirl.

Boris Johnson runs near his home on Tuesday ahead of his speech at the Conservative Party Conference. Photograph: Getty Mrs Foster praised Mr Johnson’s “belief” and “spirit” and said she’d be happy to work with him as prime minister.

“People want hope, they want to be positive,” Mrs Foster told the newspaper.

She also said she would look at any new offer to the European Union on the Irish border, but Northern Ireland must leave on the same terms as rest of Britain.

It was separately reported on Monday British Prime Minister Theresa May said she is preparing to make a new offer on the border aimed at breaking the deadlock in the Brexit negotiations.

“We should wait to see what the text actually says in relation to any offer that has been made to the European Union,” Ms Foster said.

“We always said that we have just the one red line - we cannot be separated from the rest of the United Kingdom, from a constitutional position and also from an economic reason as well.”

‘A dose of reality’ Meanwhile, the DUP’s Jeffrey Donaldson said fixation on the Irish backstop is getting in the way of a deal that would be good for everyone.

If people stopped being so pessimistic and put half as much effort into entering a trade agreement between the UK and the EU “then we would all be better off,” he told RTÉ’s Morning Ireland.

Responding to Mrs Foster’s remarks saying she could work with Boris Johnson if he became UK prime minister, Mr Donaldson said she had said there was “a lot of solidity in what he said,” but that did not necessarily mean she supports him.

It all depends on the outcome of the Brexit negotiations, if there is a no deal scenario, then there will have to be a change in how north/south institutions operate, he said.

Mr Donaldson said he wants to see the Belfast Agreement continue. Ms Foster had been reflecting the reality of what could happen in a ‘no deal’ scenario.

“We would be deluding ourselves if we think that (no deal) would not change the way we do business.”

He gave the example of the single market for the island of Ireland. If there is a no deal outcome then that will have implications and adjustments will have to be made.

In response to a Times of London report that Mrs May will put forward new proposals on the backstop which would involve the UK remaining tied to the EU’s customs rule book beyond the two-year transition, and until a mutually acceptable technological solution on the Irish Border could be found, Mr Donaldson said that such a proposal had not been put to the DUP.