On ConservativeHome today, our columnist David Gauke, formerly a pillar of Remain, makes a case for No Deal.

Elsewhere, there is a vogue for claiming that the latter (or an “Australian-style deal”, as we’re now meant to call it: in other words, the most basic of WTO terms) is so like a Canada-type deal that Boris Johnson will actually go for it.

Maybe. But there is a green fly in this red, white and blue ointment: Ireland.

On paper, there must be a border with the EU.  And it must be drawn either in the Irish Sea between Great Britain and Northern Ireland, or on the island of Ireland between the United Kingdom and the Irish Republic – in other words, on Ireland’s land border.

The position in the Withdrawal Agreement is clear.  The border will be in the Irish Sea.  However, the EU and the Government and the UK disagree about the consequences.

The EU says that the Northern Ireland Protocol necessitates border checks on good moving from Great Britain to Northern Ireland.

The UK counters that the implementation of the Protocol remains subject to discussion and agreement – and is ruling out intrusive checks.

And exactly how trade across the Irish Sea will works will be determined by decisions yet to be taken in a UK-EU Joint Committee, and by the wider UK-EU trade relationship – the main subject of stage two of the Brexit talks.

So, then: in the event of No Deal, goods will flow from Great Britain through Northern Ireland – and thus unregulated into the EU’s Internal Market.

Broadly speaking, an EU coalition led by the Commission and France will prioritise the protection of the market, while Ireland will be focused on preserving the present settlement on the island.

There are four broad possibile outcomes.

First, pressure from the coalition leads, sooner or later, to a rough and ready land border in Ireland.  Leo Varadkar’s government hated the idea.  So did Irish public opinion.  Varadkar has now gone and Sinn Fein, the greenest of all Ireland’s party, and one with a paramilitary wing to boot, are on the rise.

Our own security services didn’t like the idea of a Brexit land border before Boris Johnson’s deal and Sinn Fein’s rise.  They will like it even less now.  (Furthermore, there is no precedent for checks between two parts of the Single Market – since the Irish Republic and Northern Ireland will be in both).

Which leads to the second possibility.  The Prime Minister U-turns and slaps intrusive controls on goods moving from Great Britain to Northern Ireland – bowing to the ultimate logic of the Withdrawal Agreement, which of course would remain in place.

Business in Northern Ireland would complain.  So would Ulster’s Unionists.  And protest in Northern Ireland doesn’t always come in a green jumper.  Sometimes it wears an Orange one.

So Johnson will continue to insist that he won’t do anything of the kind – just as he insisted in 2018 he wouldn’t put a border in the Irish Sea at all.  But that was before he had a near-landslide tucked under his electoral belt.

Third, Ireland eventually decides that checks on the island of Ireland is even more painful, at least in political terms, than checks between Ireland and the EU.  And that the least unpleasant option is first to put these in place, and then blame the Brits.

Some Brexiteers talk up this possibility, but it flies in the face of Ireland’s politics, economic development (at least in most sectors) and culture.  And why would other parties in Ireland want to create what would be a propaganda gift for Sinn Fein?

And finally…Ireland gets its way.  There is a bit of leakage into the Single Market but the French, grumblingly and relucantly, consent to connive in turning a blind eye.  Everyone muddles through.

So much for the options.  They give reason to pause and ponder.  It was the Irish dimenson – not money, not citizens – that proved the biggest obstacle to negotiating the Withdrawal Agreement.  By extension, it’s therefore one of the most formidable to a No Deal outcome.