The Taoiseach must block progress of the Brexit negotiations in December if the UK fails to give a formal written guarantee that there will be no hard border with Northern Ireland, Sinn Féin leader Gerry Adams has said.
With a pre-Christmas election now averted, Leo Varadkar's focus will be firmly on crucial Brexit negotiations in Brussels on December 14 and 15.
UK prime minister Theresa May has rejected Mr Varadkar's demands for written commitments before trade talks begin to avoid checks on the Irish border, even if it that means Northern Ireland continues to apply EU trade rules while Britain diverges.
This has prompted Mr Varadkar to threaten to veto the start of trade talks at the summit.
Speaking on Tuesday Mr Adams said that Mr Varadkar must use the veto.
"Brexit in British Tory terms is bad for every single person on this island and bad for the economy. We need to be standing very firm. The North needs to be kept in the European Union," the outgoing Sinn Fein President said.
He also said that the Democratic Unionist Party (DUP) was "totally irrational and illogical" in their Brexit position and accused them of being "disrespectful that the people of the north voted to remain and that should be upheld".
Referring to the collapse of the Stormont powersharing government, he said even if the Executive was restored the differences between Sinn Fein and the DUP "will prevail".
"If we managed to get the institutions back in place there is a chasm between our position and the DUP position. The DUP ignores the consequences (of Brexit) for people, many of them Unionist people," he added.