There is a magnificent paradox – the Taj Mahal of paradoxes, let’s hope NOT the RMS Titanic of paradoxes – in the opposition of Northern Ireland’s DUP to Boris Johnson’s Brexit.

Johnson’s replacement to the backstop, by design, keeps the province much more closely aligned with the tax and business rules of the EU than would be true of Great Britain. It does so in order to keep the border between Northern Ireland and the Republic of Ireland free of friction and free of opportunities for smugglers and terrorists to return to their toxic ways of yore.

For the DUP, this alignment introduces a fat new border between NI and GB, in the form of customs and regulatory checks in the Irish Sea. And of course it is wholly understandable that any such border will trouble unionists.

But it is also that alignment which means Northern Ireland’s economy and prosperity will be much more influenced by the economic performance of the Republic of Ireland and the EU than would be true of the rest of the UK.

So here are the horns of the DUP’s dilemma.

Remember that the DUP is a Brexit-supporting party. And presumably it therefore believes that Brexit will make GB relatively richer than the EU.

Surely therefore DUP MPs should be confident that in five years or so, Northern Ireland’s people will be utterly fed up of being so tied to the fortunes of the supposedly sclerotic EU economy and will therefore order their representatives in the Northern Ireland Assembly to prize the province free of the EU’s web or constricting rules and regulations.

And under Johnson’s plan, the Assembly will have the right and duty to decide whether to make that rupture with the EU.

But what the DUP appears to be arguing, implicitly, is that there is too much of a risk that Northern Ireland’s people will become relatively richer as a result of their continued intimacy with the Republic and the EU, and therefore will never want the true Brexit divorce.

Which, as I say, is an odd argument for a Brexit party to make.

And it is doubly odd since the province is literally the only part of the UK that is benefiting from Johnson’s notorious cakeism, with its dual membership both of the UK’s customs area in law and the EU single market in practice.

As a result, NI would be able to feast on all the chlorinated American chicken it can stomach, once Johnson has his sparkling trade deal with Trump’s America. And it would also be able to sell engineering components to the rest of the EU, without the costly friction of border checks.

No other part of the UK would be able to have and eat both those cakes..

And Northern Ireland would have the right to cut its VAT rates to those of the Republic, below GB rates – which the DUP may feel is scandalous, but is presumably not something they will shout too loudly about.

Perhaps the biggest cause of unease for the DUP is that there would be no deal for MPs to vote on without the intercession in the Wirral last week of the Irish PM, Leo Varadkar, when he made a massive concession to Boris Johnson.

Very much against the consensus view in the EU, Varadkar agreed with Johnson to give Northern Ireland’s Assembly the right to rip up the new arrangements for NI on a permanently rolling basis.

Varadkar took a significant personal risk in dismantling the roadblock to a Brexit entente by conceding that the people of NI must be able to choose to break more fixedly from Brussels, if that is their future preference.

London tells me such. Brussels tells me such.

But just because the deal is made in Dublin does not mean it is a cunning plan to break up the United Kingdom.

In fact a no-deal Brexit – the theoretical alternative to Johnson’s deal – is perhaps more likely to lead to the secession of Northern Ireland from the UK, because it would cause constitutional, political and economic mayhem, and precipitate a border poll or referendum under the terms of the Good Friday Agreement.

The DUP bridles that it has not been given the power to veto the new arrangements for NI from now till eternity. And having propped up the limping Tories in government for more than two years, it is obvious why that feels like Johnson’s cruellest insult.

The DUP should have remembered the Conservatives’s long history of chewing up and spitting out their partners in government. Or they could have asked the Lib Dems.

And if the DUP wants revenge, forcing a no-deal Brexit on a Northern Ireland that voted to Remain may feel like a face-saver but could be self-immolation.

Robert Peston is ITV’s Political Editor. This article originally appeared on his ITV news blog