The difficulty for other governments in dealing with Boris Johnson is to figure out whether he is lying or merely ignorant. There was so much weirdness in the general election campaign that it was easy to miss a moment that would have once caused something of a sensation. But in this new era, it was barely remarkable that a friendly foreign government had to intervene to say that important statements by a British prime minister were patently untrue.

On the weekend before polling day, Johnson told Sky News that there was “no question” of checks being needed on trade between Northern Ireland and the rest of the United Kingdom under his withdrawal agreement. The Irish deputy prime minister, Simon Coveney, pointedly and publicly refuted this absurd claim: “Goods coming from Great Britain into Northern Ireland will need to have some checks to ensure that the EU knows what is potentially coming into their market through Northern Ireland.”

It is deeply unsettling for the unionist community. No good can come for any settlement from their sense of abandonment

Nobody could say for sure whether the prime minister knew what he was saying was false, or whether he doesn’t actually understand what he signed up to. The one thing we can be certain about is that Johnson reckons that most voters in England don’t care one way or the other. In that, he is surely right.

In Ireland, as in most other European countries, there is some relief that Johnson now has a majority large enough to push through the withdrawal agreement. Nobody buys the notion that Brexit will be “done” at any time soon, but at least we are approaching the end of the vastly over-extended first chapter. These latest bogus claims from Johnson, however, point to two big problems for both the Dublin government and for a renewed administration in Belfast, if one is formed as a result of talks beginning this week.

The first big problem is that Johnson’s thumping majority doesn’t make his mendacity and fecklessness go away. The Irish government wants the withdrawal deal to be ratified, largely because it got what it wanted: a version of the Northern Ireland-only backstop. But this is like inheriting valuable shares in a very dodgy company. Johnson’s actions and attitudes will still matter greatly to both parts of Ireland and, although it did not matter to many voters last week, his approach to Ireland remains, to put it charitably, cavalier.

This was not the first time Johnson had peddled such blatant falsehoods about what he has signed up to in the withdrawal agreement’s Irish protocol. On 8 November, in an official briefing for the parties in Northern Ireland, he said: “There will not be checks on goods going from Northern Ireland to Great Britain.” This, too, is either a lie or a manifestation of utter ignorance – his own Brexit secretary, Steve Barclay, acknowledged that goods going from Northern Ireland will have to be accompanied by exit declarations and “targeted interventions” from customs officers.

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This spoofery has to be seen in the context of Johnson’s reneging on his absolute commitment to his former allies in the Democratic Unionist party that a “border in the Irish Sea” is something “no British government could or should” ever accept. The DUP may have been very stupid to trust Johnson, and it is easy to feel that they got what they deserved. It may also be true that the “border in the Irish Sea” is the least harmful way of avoiding the even worse option of a hard border on land. But neither of these considerations takes away from the obvious truth that the prime minister’s dealing on this whole question has been opportunistic and unprincipled. He seduced the DUP when it suited him and ruthlessly dumped them when it didn’t. Only a fool would assume he won’t do the same to any other Irish party or government in the future.

The second big problem is that all the evidence suggests that Johnson has no real knowledge of or interest in the consequences of the withdrawal agreement – and of Brexit itself – for Northern Ireland. He is pushing through a huge change in the nature of the United Kingdom – one part of it is effectively being jettisoned as dead weight so that the great Brexit balloon can ascend into the blue skies of renewed greatness. Whether one regards this as a good thing or a terrible betrayal, it is a matter of great long-term consequence for both islands. Apart from everything else, it is deeply unsettling for the unionist community in Northern Ireland. No good can come for any future Irish settlement from their sense of abandonment.

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Any responsible political leader would prepare the people most affected for such a big change – Johnson didn’t. But it’s much worse than that: he keeps, in effect, denying that it is really happening. His false claims about the withdrawal agreement are part of his wider rhetorical pitch: that, as he put it again in his victory speech on Friday morning, the UK is “leaving the EU as one United Kingdom”. Even if we ignore the Scottish question, this is utterly fraudulent. It is a matter of fact that Northern Ireland is not about to leave the EU on the same terms as Britain.

In this case, we can rule out the possibility that Johnson simply doesn’t know this. He’s lying. This, of course, is no longer newsworthy. But it is reprehensible – British citizens in Northern Ireland have a right to be told by their prime minister the truth about their future. Johnson’s indifference to that necessity is bad news for all of Ireland.

• Fintan O’Toole is a columnist for the Irish Times and author of Heroic Failure: Brexit and the Politics of Pain