The UK must seek and the EU must support a "generous special status" for Northern Ireland post Brexit, says the Republic of Ireland's opposition leader.
Micheál Martin called Brexit "one of the biggest mistakes of modern European democracy".
Ireland now faces "a profound threat" and needs EU solidarity, he said.
And he called on London to acknowledge that Northern Ireland is "a special case", and must be treated accordingly.
Addressing a meeting at Queen's University, Belfast, Fianna Fáil leader Mr Martin claimed the continued failure to have a proper civic dialogue on Brexit was a clear breach of commitments under the Good Friday Agreement.
He told Thursday's audience that there was "no evidence whatsoever" of a credible plan from the UK government for what happens next.
The period from now to March 2019 would define the future direction for decades to come, he added.
Everyone would lose if, post-Brexit, the UK "drifts off into the night", he said.
"The EU also needs to show more urgency and ambition in reforming its own workings."
On the Republic of Ireland's position, Mr Martin said: "We are faced with a profound threat because of the actions of another state on which we had no influence.
"We are showing solidarity with the European Union and it must show us the same."
Businesses in the republic were already suffering because of the dramatic fall in sterling, he pointed out.
Mr Martin said imposing the full impact of a hard Brexit on Northern Ireland was "unacceptable".
"Dublin must promote and support special status for Northern Ireland in whatever way possible," he said.
He pointed out that Northern Ireland would contain the largest concentration of EU citizens living anywhere outside the boundaries of the EU.
A clear majority of Northern Ireland voters opting to Remain could not be lightly dismissed, he argued.
He said he would oppose any proposal to force people in Northern Ireland to choose between EU and UK citizenship.
Good Friday breach?
Mr Martin was highly critical of the exclusion of the Northern Ireland secretary from permanent membership of the UK's Brexit key negotiations committee.
Speaking about the current situation between Northern Ireland and the Republic of Ireland, he said: "The continued failure to have a proper civic dialogue is a clear breach of commitments under the Good Friday Agreement and it is at moments such as this that its absence is most felt.
"Northern Ireland is a divided and increasingly diverse society," he said. "A complex challenge such as this needs the legitimacy of ongoing civic engagement - something even the Office of the First Minister and Deputy First Minister would admit has been lacking."
The Irish opposition leader said: "The scale of the direct economic, social and political threats posed by Brexit far outweighs its opportunities."
This is "one of the most important challenges ever faced by governments on these islands," he added.