I have a confession to make. Boris Johnson and I have a quite a bit in common. We attended the same Oxford College (Balliol). We studied the same subject (classics). We were presidents of the same debating society (the Oxford Union). However, a crucial difference is that I gave up being an undergraduate when I left university four decades ago.

As Johnson is considerably younger than me we were not contemporaries at university. However, when later I was the Irish ambassador in London and he was the mayor, we met a few times. As might be expected he was charming and ebullient. There was a touch of mischief in the Johnson eye. The trademark awareness that he was an actor in his own play, a charisma looking for an identity.

He once accepted an invitation to dinner at the embassy. His attendance was confirmed by his office on the morning of the event, but he pulled out at the last minute. People, of course, often have to cancel appointments, so this was entirely insignificant in the scheme of things. However, it is just possible that Johnson’s cancellation may have had a slightly deeper significance.

I was subsequently informed that he often used to accept two clashing invitations and only decide at short notice which event to attend. I can’t be certain that this is true, but it seems plausible. A similarly whimsical approach to entering commitments found an echo in the famous manner of Johnson’s announcement of support for the leave campaign. It now seems accepted that, having prepared one article in support of Brexit and another against, he only decided which line to commit to at the last minute.

Johnson’s discourse and his writing suggest some appreciation of Latin and Greek. But classics, like every academic discipline, has at its heart the idea that there is something called the truth which we are striving to find. In studying Thucydides, for example, we might disagree about the reasons for the Peloponnesian war, but we study it because we believe there are truths worth exploring. Virgil, like any poet, is open to different interpretations, but we read him because he is grappling with important verities. The reason for reading Homer and Horace goes much deeper than memorising classical tags to provide a veneer of culture in later life.

To be fair, I don’t believe that Johnson’s fatuous claim about extra Brexit money for the NHS or his glib dismissal of the profoundly important Irish border issue should be understood as lies. To tell a lie, one must first understand that there is such a thing as truth. Mere frivolousness about matters that will impact on ordinary people, especially in the UK itself, seems to me to be a greater threat. Although, of course, I’m not British, I find it hard to believe that anyone lacking in seriousness of purpose could purport to regard Winston Churchill as a hero.

The flip-flop about whether the UK would effectively have full trade access to the EU after Brexit has been at the heart of the Johnson nonsense about having his cake and eating it. It is now increasingly clear that, far from British business retaining untrammelled access to the internal market, as claimed during the referendum campaign, the metaphorical cake mysteriously disappeared over tiffin at Johnson’s London club. Having your cake and eating it is prevented by the laws of physics, not by the laws of Brussels.

Oxford has a reputation for tolerance. Personally I encountered nothing but respect and friendship when I arrived there at the height of the Northern Ireland Troubles. Among the many who are saddened by Johnson’s willingness to ride the populist wave can, I think be counted, most people associated with the university.

I make these points reluctantly. I do so because, after a difficult history, Ireland and the UK have developed a deep and respectful friendship largely in the context of our shared membership of the EU. Honesty is an essential ingredient of friendship. So is a seriousness about things that matter to one’s friends. Personally, I disagree with a great deal of what is said in the Brexit debate in the UK, but I recognise a willingness across much of the political spectrum to treat the issues seriously.

Thucydides wrote that most people will not take the trouble of finding out the truth but are more inclined to accept the first story they hear. Watching Johnson hold forth to his faithful when he dropped into the Tory party conference last week, it struck me that he may have picked up something at university after all.

• Bobby McDonagh was Ireland’s ambassador to the UK from 2009 to 2013