The convention is that a leader’s speech at a party conference is a serious account of domestic and foreign policy priorities. Boris Johnson characteristically defied that convention in Manchester. His address was trivial and insubstantial, perhaps deliberately so in recognition that the more consequential event was publication of proposals for a new Brexit deal.

The draft deal is meant to address the problem of the Irish backstop, which Mr Johnson has declared unacceptable but which, according to EU leaders, can be replaced only with legally watertight alternatives. The proposed solution is interlocking jurisdictions, with Northern Ireland as a special “zone of regulatory compliance”, aligned with parts of the European single market, while also part of a UK-only customs regime. The problem of future divergence is kicked down the road. Northern Ireland’s unique status would be routinely reapproved by the Stormont assembly.

It is not clear how any of this will satisfy concerns that Dublin and the rest of the EU have repeatedly raised about the integrity of the single market, and the Good Friday agreement. Mr Johnson’s offer makes explicit his intention to effect a significant rupture from EU legal norms. He concedes that his plan requires a new customs border between Northern Ireland and the Republic, but insists that checks can be enforced in ways that are not disruptive, or damaging to the peace process.

This is an old fantasy peddled by hardline Eurosceptics, based on the notion that technology can monitor goods remotely and invisibly. That is true of shipments that are declared in accordance with the rules. It doesn’t deal with smugglers and terrorists who do not comply with the rules.

Mr Johnson’s plan would set up a hazardous future confrontation over Northern Ireland’s economic orientation. What if Stormont fails to settle the alignment question? The assembly has been suspended for more than two years. Ambiguity around that mechanism implies Brussels losing control of an external border of a single market. That is something the EU would not tolerate. Mr Johnson has not considered the concerns of all of Northern Ireland’s communities. He appears to have consulted only one faction – the DUP. That party does not represent the majority view with regard either to Brexit or the Good Friday agreement, which it rejected in 1998.

Mr Johnson’s plan does not look like a credible formula for stable partnership with the EU but, rather, a jumble of half-baked plans. It is worrying to note that this is the kind of deal a cynical prime minister might present without expectation of success, but with the intent of whipping up an electoral base with the claim that he offered a deal and was rebuffed by foreigners.

In his speech, Mr Johnson suggested that the only sticking point might be “a technical discussion of the exact nature of future customs checks”. If he believes that, he is dangerously ignorant of Irish history. If he doesn’t, his belittling of substantial concerns is a deliberate insult to those in Brussels who are negotiating in good faith. He said nothing original in the area of domestic policy, nothing meaningful about economics and nothing at all about foreign relations, beyond the dubious assertion that leaving the EU facilitates trade with other countries.

His urgency about the 31 October Brexit deadline was expressed as an impatience to escape discussion of Europe. The prime minister’s argument appears to be that the UK must hurry up and complete a revolution in its economic and strategic orientation so as to avoid any serious interrogation of the benefits and purpose of that revolution. This is a dishonest position because the moment of legal separation from the EU, with or without a deal, marks only the beginning of an arduous negotiation. The prime minister’s own plan raises more questions than it answers about future relations with the continent. Mr Johnson says he wants a deal. Perhaps he does. But his Brexit priority is plainly the management of domestic opinion ahead of an election, when it should be managing the UK’s national interest in relations with neighbours.