Ireland's Taoiseach Leo Varadkar and European Council President Donald Tusk in Dublin | Paul Faith/AFP via Getty Images Donald Tusk: EU will back Ireland’s Brexit progress decision ‘If the UK offer is unacceptable for Ireland, it will also be unacceptable for the EU,’ European Council president says.

In a bluntly worded warning to London and a show of unity with Dublin, European Council President Donald Tusk said Friday that Ireland would have an absolute veto over the declaration of "sufficient progress" in the Brexit negotiations.

Speaking with Tusk after a joint meeting in Dublin on the state of play in the talks, Irish Prime Minister Leo Varadkar said that Ireland and the EU were prepared to "stand firm" and refuse to advance the talks "if the U.K. offer falls short" on any one of the three main divorce issues.

After an hour-long discussion, Tusk and Varadkar expressed guarded optimism that a deal with the U.K. could be reached in time for EU27 leaders to make a declaration of "sufficient progress" at a European Council summit in Brussels later this month. But they also made clear that the pieces of a deal were not yet fully in place.

Tusk has set an "absolute deadline" of Monday for the U.K. to come back with a revised offer on the divorce terms, particularly on the financial settlement, on which there had been little progress before the October European Council summit, and on issues related to the Northern Irish border.

U.K. Prime Minister Theresa May is expected to present the revised package during a lunch in Brussels Monday with Commission President Jean-Claude Juncker.

In Dublin, Tusk sent a firm message that first and foremost it would be for the Irish government to decide if there has been sufficient progress on Ireland border issues, including provision to protect the Good Friday peace agreement.

Varadkar cautioned that the U.K. still had to come forward with clearer proposals about how a hard border would be avoided.

"I will consult the Taoiseach [prime minister] if the U.K. offer is sufficient for the Irish government," Tusk said. "Let me say very clearly: If the U.K. offer is unacceptable for Ireland, it will also be unacceptable for the EU."

He added, "I realize that for some British politicians this may be hard to understand. But such is the logic behind the fact that Ireland is the EU member while the U.K. is leaving."

Varadkar, whose relations with the U.K. have been uneasy at best, said that there had been "some good progress" on all three of the divorce issues. His carefully worded statement stopped far short of the "sufficient progress" that the EU has insisted all along would be needed to close phase one of the negotiations and enter phase two, focused on a transition period and on the framework of a future trade relationship.

"There has been some good progress in relation to the U.K. financial settlement, and also EU citizen rights, and these issues are crucial for all EU member states, including of course Ireland," Varadkar said. "And I very much hope that the work that remains to be done can be completed in time."

He continued, "On the issues that are specific to Ireland, there has been some good progress, particularly in relation to the Common Travel Area and all the rights associated with it, and toward a common understanding of what's needed to protect the Good Friday Agreement in all its parts including of course, and crucially, avoiding a hard border."

But he cautioned that the U.K. still had to come forward with clearer proposals about how a hard border would be avoided.

EU and U.K. negotiators appeared to gain some ground there, with the British side expected to brief May so that she could make final political decisions about the offer to put forward on Monday.

"The European Union is a family and families stick together" — Leo Varadkar

"On the question of the border," Varadkar said, "as I have said many times, the best and most obvious solution would be for the United Kingdom to remain in the customs union and the single market. But as the British government has ruled that option out, it must offer credible, concrete and workable solutions that guarantee there will be no hard border whatever the outcome of the negotiations and whatever the future relationship between the U.K. and the EU is."

Vardkar, in his remarks, also included a not-so-subtle slap at the U.K., by thanking the rest of the EU countries for their strong support of Ireland.

"We appreciate the solidarity of all of our EU partners from the EU institutions and also the other member states who have taken Ireland's unique concerns to heart, and Irish concerns are very much Europe's concerns," he said. "It's an important and impressive display of European unity and solidarity. The European Union is a family and families stick together."

Except, of course, when there's a divorce.