Taoiseach (Leader) Leo Varadkar has said he believes Sinn Fein MPs should take up their seats at Westminster before the December 11 vote on the Brexit deal.

The deal was signed off by EU leaders on Sunday but faces strong opposition at Westminster, including from Northern Ireland’s 10 Democratic Unionist Party MPs who are supposed to support Prime Minister Theresa May’s Tory government.

May is attempting to combat the opposition from MPs by going directly to the people. She was in Northern Ireland on Tuesday.

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Varadkar said that for “different reasons, Sinn Fein is not participating in parliamentary democracy in Westminster or Stormont.”

He said if they are not willing to take up their seats, or feel they cannot because of their policy on abstention, “they do have the option of resigning,” to allow people in the seven constituencies in question to decide whether or not they want to “have a say” when the vote comes to Westminster.

Ireland's Taoiseach Leo Varadkar.

Ireland's Taoiseach Leo Varadkar.

The taoiseach also said the Brexit deal agreed between the U.K. and EU last weekend was the “best deal that was available to the United Kingdom.”

But he said whether or not they accept it “is a decision for the British Parliament.”

He said, “We don't want to do anything counterproductive by being seen to interfere in what is a decision for British Parliament.

“You can imagine the reaction that might happen in Ireland if you had politicians from Spain or Britain or Denmark coming to Ireland to tell us how we should vote when it comes to parliamentary vote.”

However, he said what Ireland could do is answer questions if anyone wants to ask from the U.K. and give assurances.

Sinn Fein leader Mary Lou McDonald said her party's seven votes at Westminster “will hardly be the deciding factor” for passing the Brexit deal.

She said it was ridiculous to think that Sinn Fein MPs would go to the House of Commons “on a white charger” and save May.

Sinn Fein leader Mary Lou McDonald.

Sinn Fein leader Mary Lou McDonald.

McDonald told Sean O'Rourke on RTE that the reality is that Brexit will happen -- whether on May’s watch or under Labour leader Jeremy Corbyn.

She said Westminster looks after specifically English and British interests and Sinn Fein’s presence there would achieve nothing.

May’s visit to Northern Ireland came the day after President Donald Trump said the withdrawal treaty could hamper trade deals between the U.K. and the U.S., something which May’s Downing Street has denied.

Trump also said that it sounded as if the plan was a good deal for the EU rather than the U.K.

While in Belfast, May said, “This deal avoids a hard border between Northern Ireland and Ireland. This has been at the forefront of my mind throughout the negotiations.

"It has been especially clear to me when I have visited communities along the border in Northern Ireland and seen first-hand how important it is that the unique circumstances local employers face are recognized in any agreement.

"They need to be able to trade freely across the border with Ireland and have unfettered access to the rest of the United Kingdom's market.

"This deal makes that possible and that's why, across Northern Ireland, employers large and small have been getting behind it."

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British Prime Minister Theresa May.SkyNews