Gerry Adams has called on Taoiseach (PM) Enda Kenny to stand up for Ireland's "national interests" ahead of the UK triggering the Brexit process.
Prime Minister Theresa May is to officially notify the European Union next Wednesday that the UK is leaving.
Sinn Féin leader Mr Adams said Mr Kenny must work to secure special designated status for Northern Ireland within the EU.
Northern Ireland voted to remain in the EU referendum by 56% to 44%.
"The taoiseach has previously expressed impatience over the lack of clarity from Britain on Brexit," said Mr Adams.
"He now knows the date on which Article 50 will be triggered. The clock is ticking, so he needs to act accordingly and stand up for our national interests across the island, at European level, to secure special designated status for the north within the EU.
"That is the consensus of the majority of parties north and south.
"If the British government succeeds in its plans, it will drive part of Ireland out of the European Union.
"That is not acceptable."
The party's northern leader Michelle O'Neill said she planned to meet Mrs May to voice her strong opposition to the triggering of Article 50.
The prime minister is reportedly set to travel to the region later this week, coinciding with the last few days of talks to form a new power-sharing executive at Stormont.
"The British government are acting against the interest and the expressed wishes of a cross-community group of people here that want to stay in Europe," said Mrs O'Neill.
"We will be making that message clear to Theresa May when she is here - apparently she is coming later in the week - we will make this message very clear to her, that this is not good, this is bad news for the people of Ireland."
'Ready and waiting'
Earlier, Downing Street said Mrs May would write a letter to the European Council, adding that it hoped negotiations on the terms of Brexit and future relations could then begin as quickly as possible.
An EU spokesman said it was "ready and waiting" for the letter.
Mrs May's spokesman also rejected reports that an early election might be held, saying: "It's not going to happen."
Under the Article 50 process, talks on the terms of exit and future relations are not allowed until the UK formally tells the EU it is leaving.
If all goes according to the two year negotiations allowed for in the official timetable, Brexit should happen in March 2019.