The Republic of Ireland's Prime Minister Micheal Martin has held talks with Northern Ireland's political leaders in a bid to defuse a growing stand-off over post-Brexit trade.

Earlier this week, UK Prime Minister Boris Johnson tried to find common ground between the Unionists led by the Democratic Unionist Party (DUP) and the Irish nationalists led by Sinn Fein.

The UK government's pledge to ditch the Northern Ireland Protocol agreed as part of its Brexit divorce deal with the European Union (EU) may have pleased Unionists, but it threatens to derail a future British trade deal with the United States.

Martin was welcomed to the province by Northern Ireland's first minister designate, Sinn Fein Vice President Michelle O'Neill.

"At a time when democracy is being denied – at a time when the DUP are trying to prevent the facilitation of an executive being formed, one that can start to deliver for the public – I think it's important that he's here," said O'Neill in pointed remarks.

Sinn Fein, a party committed to pushing for a future united Ireland, came out on top in regional elections earlier this month – with a campaign focused on addressing a growing cost of living crisis.

Their unionist opponents are continuing to block a power-sharing deal – meaning the victors cannot start trying to deliver on what they promised.

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Those on the Sinn Fein side of the caretaker government have become increasingly frustrated through the week. According to Conor Murphy, Northern Ireland's Finance Minister: "We now have £410m available for supporting people – we want to get it out to people, we want to get a three-year budget in place: for healthcare, to support people. We can't do any of that."

At the heart of the issue is the EU customs border created in the Irish Sea as part of the Brexit deal. On Wednesday UK Foreign Minister Liz Truss said her government might try to unilaterally override the protocol, which it enthusiastically signed up to less than 18 months ago.

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This reassurance to the Unionists hasn't persuaded them to retake their seats in the Northern Ireland Assembly, although its leadership did respond positively.

DUP leader Jeffrey Donaldson said: "We've waited a long time to see the government that represents action to undo the problems created by the Irish sea border – the harm that it's doing to our economy".

In Brussels, the reaction has been quite the opposite, with the EU accusing London of blackmail. A visiting group of U.S. senators agrees, bringing a message that a U.S.-UK trade deal is off the cards if London pushes ahead.

It's a conundrum with no easy answers – and the impasse has left the very viability of Northern Ireland's fragile political order in doubt.