The shape of the Brexit fix that Boris Johnson wants from the EU’s 27 leaders is now clear. Here it is:

In place of the dreaded backstop – that insurance policy for keeping open the border on the island of Ireland hated by most Tory Brexiters and Northern Ireland’s DUP – Johnson is suggesting:

a) A unified single market for agriculture between Northern Ireland and the Republic (a single set of what are known are sanitary and phytosanitary rules), so that cross border flows of livestock and food is not hindered;

b) Customs and limited unintrusive goods standards checks on the island but away from the border itself;

c) No customs union with the EU for either the whole UK or NI alone;

d) Where rules for agriculture or even for other limited markets are set for the whole island by Brussels, the principle of a “Stormont lock” – or, in the words of a source, that “the people of Northern Ireland must be able to withdraw consent, with all that entails”.

There is lots to say about all this. But the biggest and most important question is whether Brussels and the EU27 will – and can ever – accept the principle that the citizens of Northern Ireland could unilaterally choose to end the arrangement. This is an absolute must for Johnson, I am told.

Equally, Brussels has always insisted that any arrangement to keep open the border should not be capable of being terminated by one side only. For what it is worth, Johnson believes he is getting a fair hearing in Dublin.

But that is a long way from saying that his Irish counterpart could or would accept Johnson’s plan. And even if Varadkar is persuadable, it is not a given Varadkar would be able to convert the rest of the EU – because although EU leaders have said they will never abandon the Republic of Ireland, that is not a blank cheque from Varadkar to do whatever he wants to protect his economy and security, at the potential expense for the rest of the EU of holing their single market and creating what they would see as a fatal exception to their rules.

Many of you will say that what Johnson wants is wholly unrealistic. And that therefore on his watch, a no-deal withdrawal from the EU looms. Which is probably right.

But it is significant that EU leaders have ceased saying that in no circumstances would they open up and change the Withdrawal Agreement they negotiated with Theresa May.

Also Northern Ireland’s DUP – the unionists still propping up Johnson’s numerically challenged government – seem to be open to accepting more Brussels-generated rules for an all-Ireland economy than seemed remotely possible only a fortnight ago.

We should know in not much more than a week whether Dublin is prepared to argue to Brussels whether there is the basis for a reworked Brussels agreement, and then whether Brussels will be amenable to any lobbying by Varadkar.

For what it’s worth, Johnson is more optimistic of striking a deal than his officials and advisers – which is largely to do with the PM’s nature, and theirs.

If I am cautious, and fear the gloomier officials are correct, it is because the EU is a rules-based endeavour or it is nothing. And I struggle to see how even Johnson’s sunny cajoling can create the Northern Ireland exception he craves.

Robert Peston is ITV’s political editor. This article originally appeared on his ITV News blog