When people are screwing up, they tend to take their rage and frustration out on their nearest and dearest. If, as seems increasingly likely, the European Union summit on 15 December does not give the go-ahead for talks on a post-Brexit trade deal, we already know who’s going to get the blame.

It will be all Ireland’s fault. The Sun this month gave the taoiseach, Leo Varadkar, fair warning, advising him to “shut your gob and grow up” and stop “disrespecting 17.4 million voters of a country whose billions stopped Ireland going bust as recently as 2010”. Boris Johnson, in Dublin, delivered a slightly more diplomatic version of the same message. The Irish should stop worrying about a hard border being reimposed on their island, trust all the lovely reassurances they have received from the British government and make the necessary declaration that “sufficient progress” has been made on the issue for substantive talks to go ahead.

This is probably not going to happen. Ireland may well be plunged into calling a snap general election this week, which in itself will make any major shift in its approach to the Brexit talks before 15 December even less likely. No possible outcome of that election will weaken Irish insistence on the imperative of avoiding a hard frontier and the need for the British to come up with actual proposals about how this is to be done.

As things stand, the December summit seems likely to say that enough progress has been made on two of the key preliminary questions, the divorce bill and the mutual recognition of the rights of expat citizens.

But Ireland will be the spoke in the wheel. The verbal missiles that have already been tested will then be launched across the Irish Sea. A whole country will join the ranks of saboteurs and renegades, without whom the Brexit project would be proceeding triumphantly.

To grasp the full stupidity of this situation, remember that Ireland is actually Britain’s best friend on the other side of the negotiating table. This is partly because, before the Brexit referendum, Anglo-Irish relations were warmer than at any time in the long and often bitter history of mutual entanglement. The two governments worked hand in glove on the Northern Ireland peace process and developed a genuine trust. They also co-operated very closely within the European Union. But even leaving friendship aside, Ireland has an overwhelming interest in making Brexit as painless as it possibly can be. A bad Brexit will destabilise Northern Ireland and damage the Republic’s economy, in which most small and medium-size companies depend heavily on the British market.

It is thus quite a feat for the Brexiters to turn their most sympathetic ally into the scapegoat for their own most egregious failures. They’ve pulled it off by utilising their most remarkable skill: sheer incompetence. They have known since 29 April, when the European commission issued its negotiating guidelines, that credible proposals on the Irish border were a basic condition that had to be satisfied before trade talks could start. This could not have been more explicit.

Time after time, the lead EU negotiator, Michel Barnier, has made it clear that “the unique situation on the island of Ireland requires specific solutions”. But in any case, one would expect Britain to be just as insistent. It has grave responsibilities to its own citizens in Northern Ireland and to the Belfast agreement, by which it is legally and morally bound.

Yet the British have done essentially nothing. Johnson’s referendum stump speech boasted of selling French knickers to France and boomerangs to Australia, but even he did not anticipate one of the biggest export successes of Brexit: selling blarney to Ireland. In six months, Britain has produced one flimsy paper on the border question, published in August to almost universal derision.

Facebook Twitter Pinterest Boris Johnson said Ireland shouldn’t worry too much about the border during his visit to Dublin. Photograph: Reuters

It claims there will be no hard border – indeed, no physical border infrastructure whatsoever – because the EU is going to agree a lovely free trade agreement with Britain that will be just as good as the single market and the customs union. Asked about this document by the public accounts committee at Westminster last week, the best the head of Her Majesty’s Revenue and Customs, Jon Thompson, could manage in its defence was that “opinions vary” as to its merits.

At that same hearing, the woman charged with planning for Britain’s post-Brexit borders, Karen Wheeler, was asked an apparently soft question: “Obviously, we have the situation of Northern Ireland and the land border there; we have 300 crossing points where people and goods can freely move. From your point of view, in your team’s planning, what are the specific challenges associated with planning for these changes between the UK and southern Ireland?”

Her reply was breathtaking: “I am not really able to say. That area is not within the scope that we in the border planning group have been working on. The arrangements on Ireland are still subject to negotiations and ministerial discussion, so that has not come within our scope at this stage.”

What Wheeler was saying is that not only does Britain not have specific plans for the Irish border – it has not even begun to consider what those plans might be. When the PAC’s chair, Meg Hillier, suggested that this was “pretty poor”, Thompson jumped in: “We need the political process to go a bit further before we can fully get into understanding it.”

The “political process” is the Brexit negotiations, in which Britain was supposed to table “specific solutions” on Ireland by October. That deadline had to be extended to mid-December. Yet here, a month before a decision has to be made, we have the most senior British officials stating openly that they still don’t understand the problem, let alone envisage a concrete solution.

So what is the Irish government supposed to do? What happens with the border is a vital national interest. Ireland is desperate to hear what Britain has in mind. Instead, it has been told not to worry its pretty little head about it, but trust in the reassurances of its betters. It is being placed in the position of a 1950s wife, whose husband is betting the house on a horse race while he tells her, with increasingly irritation, to stop worrying because the nag is sure to romp home.

Behind this reckless arrogance, there is an assumption that Ireland is an eccentric little offshoot of Britain that must shut its gob and stop asking awkward questions. It is, in fact, a sovereign country with the full backing of 26 other EU member states – and how strange it is that we have reached a point where this comes as an unpleasant surprise to so many people in London.

Fintan O’Toole is a columnist with the Irish Times