Gerry Adams has claimed Northern Ireland is not bound by the Brexit result.
The Sinn Fein leader said a majority vote in the region to stay within the European Union must be upheld.
And he told the Democratic Unionist Party – their power-sharing partners in the Stormont Assembly who campaigned for a Leave – that they must respect the Remain vote.
Urging the Dublin government to respond to the EU referendum result “on an all-island basis”, he also called for a united Ireland referendum in the time ahead.
“People voted to remain within the EU. That should be upheld,” he said.
Some 56% of the electorate voted Remain in last week’s in/out referendum and 44% backed Leave.
Mr Adams was speaking in the Irish parliament, the Dail, as it was recalled to debate the fall out of the referendum result.
“Some will say we are bound by a so-called United Kingdom vote,” he said.
“Sinn Fein says we are not.
“We need to put the island of Ireland first. We stand by the vote of the people of the north.”
Mr Adams said there is now “a huge responsibility on the Irish government to think nationally – on an all-island basis”.
Stormont ministers should be allowed to deal directly with the EU institutions, he said, through maximum cooperation between the Executive and the Dublin government.
“The Democratic Unionist Party must also respect the Remain vote,” he said.
“The majority of citizens in the north, including many unionists, rejected its exit policy. The DUP should accept this.”
Taoiseach Enda Kenny told the parliament the referendum result set off a “political earthquake”.
Ireland has much more at stake in the aftermath than any other EU country but was ready to work in its own national interests, he declared.
Top level officials are already working urgently and intensively with Britain to secure progress made by the peace process in the wake of the outcome, he told the Dail.
Dublin, Belfast and London would also press to keep the Irish border open.
“All three administrations share the common objective of wanting to preserve the Common Travel Area and an open border on the island of Ireland,” he said.
Micheal Martin, leader of the chief Opposition party Fianna Fail, said Ireland should stand ready to be a friend to Scotland after First Minister Nicola Sturgeon said it would be democratically unacceptable for her country to be forced out of the EU.
“I and my party believe that it would be unacceptable for Scotland to be treated as a normal candidate country should it seek to remain as a member of the EU,” he said.
“It currently implements all EU laws.
“It manifestly would not need to be reviewed for its standards of governance and ability to implement EU laws. It has a strong administration, a distinct legal system and an absolute commitment to European ideals.
“Scotland is strong enough to advocate for itself, but Ireland should be its friend and demand fair play should it seek to remain in the EU.”
Mr Martin said the next few years will be “a defining moment in our history and in the history of Europe”.
“The stakes could not be higher,” he said.
“We must prepare for new threats and possible opportunities.”