The Taoiseach has threatened to veto what Mrs May wants – to move on to negotiating a trade deal with the European Union in just two weeks’ time – unless firm assurances are given that there will be no “hard border”.
Donald Tusk, the European Council President stood alongside him, appeared to acquiesce declaring that the “UK’s future lies in some ways in Dublin”.
Is this one in the eye then from our closest ally, or is this row not quite what it seems?
The EU-27 must agree that “sufficient progress” has been made on the three key issues next week – EU citizens, money and the border – in order to move on to the next phase of negotiations, so clearly the clock is ticking.
Mrs May is trapped between a rock and hard place because if Britain does leave the single market and customs union as she intends, it is hard to see how the UK’s only land border with the EU – between Ulster and the Republic – can stay as it is.
Indeed the Commons Brexit Committee, whose report into this issue was published on Friday morning, came to the conclusion that a return to border checks was inevitable – although it was not backed by all the MPs.
The suggestion floated last week that the Government might get around this by allowing Northern Ireland the powers to have different customs regulations to the rest of the UK has triggered fury from the Democratic Unionist Party.
Their 10 MPs at Westminster keep Mrs May in power, and they have made clear that any such move could see them withdraw support from her fragile administration.
However unlikely it may seem that the Unionists would bring her – and their best chance of achieving Brexit – down and run the risk of propelling Jeremy Corbyn to power; senior DUP sources insist this is a real deal-breaker.
For Mr Varadkar, this is also the natural moment to push Westminster hard.
He also presides over a minority administration and needs to assert his authority. The UK Government has, after all, already caved in on the issue of the Brexit bill in the past week.
It seems like deadlock. The DUP would veto what the party see as a border in the Irish Sea, while the Republic effectively wants Britain to stay in the customs union.
The border, established in 1921, runs for 310 miles and has more than 200 public roads crossing it.
The prospect of a hard border, with customs checks to enforce the tariffs which would be needed if Britain leaves the EU’s customs arrangements, is not just a logistical and trade issue but would undermine the basis of the 1998 peace agreement.
This issue is not going to be settled finally in the next few days, and No 10 sources say they see the border entwined with trade issues which can only be fully resolved when the talks move on to the next phase.
Can Mrs May find a fudge acceptable to both sides, which would give Ireland the assurances it is seeking on the border and keep the UK Government’s options open on a customs deal?
Belfast is heavily reliant on trade with the rest of the UK and for Dublin, the economic consequences of a “no deal” Brexit could be ruinous.
Behind the scenes, both the Irish government and Westminster will have to give ground.
Both believe an agreement is do-able, but this fraught issue has raised the stakes uncomfortably high.