A very good way to judge any international action that you approve of is to try it the other way round, and see how you like it then. So, imagine that a theoretical British Prime Minister travelled to Dublin, and there made a speech at Trinity College, urging Ireland to consider leaving the EU, to make Britain’s departure from that body easier. Then imagine the same (admittedly rather hard-to-imagine)Prime Minister had also contrived to socialize in a very friendly and very public way with an Irish conservative grouping opposed to the legalisation of abortion, a divisive and active issue in that country.

Of course, no British Premier would dream of doing any such thing, which is partly my point.

There may be other possible parallels. But you can, I’m sure, work on these variations for yourselves. The argument is really about how much foreign politicians can take sides in the politics of other countries. In my experience, the rules, which should in my view be absolute, are different in practice, depending on which side the politician takes.

I personally think that such interventions as those I describe above would (on both subjects) be greeted in Ireland as at worst an outrage and at best rather ill-mannered and provocative. I also think that those reactions would be, under the circumstances, entirely justified - even though I regret Irish membership of the EU and am opposed to abortion. Because both these matters are the exclusive business of the Irish people and their chosen government.

All right, we’ve done it backwards. Now try it forwards.

Last week, the Irish Taoiseach (Prime Minister), Mr Leo Varadkar, travelled to Belfast, still just about part of the United Kingdom. As well as meeting Northern Irish politicians, he spoke at Queen’s University, Belfast.

There he made a rather powerful and pointed speech http://www.belfasttelegraph.co.uk/news/politics/taoiseach-leo-varadkar-speech-on-first-visit-to-northern-ireland-full-text-36000218.html

Personally, I have much sympathy with his fears of a hard border between Northern Ireland and the rest of the island of Ireland. But it is a United Kingdom matter, and one which we will decide. I do not think he ought to intervene in UK politics, while on UK soil.

And I think this passage

‘In my opinion, it is a tragedy of the Brexit debate that it appears that this common European identity is not valued by everyone in these islands’.

And this section ‘However, there are people who do want a border, a trade border between the United Kingdom and the European Union and therefore a border between Ireland and Britain and a border across this island.

‘These are advocates of a so-called hard Brexit.

‘I believe the onus should be on them to come up with proposals for such a border and to convince us and convince you; citizens, students, academics, farmers, business people that it's in your interest to have these new barriers to commerce and trade.

‘They’ve already had fourteen months to do so.

If they cannot, and I believe they cannot, we can then talk meaningfully about solutions that might work for all of us’…

…both go beyond the borders of legitimate concerns over joint matters, and stray into intervention in internal affairs, much as Barack Obama’s foolish and counter-productive speech on the Leave/Remain controversy did.

The attitudes of UK voters and political leaders towards any ‘common European identity’, their ultimate decisions on what trading relations they wish to have with the rest of the EU, including the 26-county state in Ireland, are their affair. If Mr Varadkar really wishes to discuss them in the hope of influencing them, it would be better if he did so in private discussions with his UK counterparts, and indeed with those who are the targets of his remarks. Indeed, if he must speak in public about them, then he would be better to do so in his own country’s universities, cities and other platforms.

Mr Varadkar was not done with interfering. He went on to attend a ‘Belfast Pride’ breakfast in the city the following day. His strong personal opinions on the issue are well-known, and Ireland voted in a 2015 referendum to legalise same-sex marriage. As above, I have myself long given up caring about this issue. But (as the recent Cake Case has reminded many) Northern Ireland is the one part of these islands in which same-sex marriage is still not permitted by law. Mr Varadkar was therefore clearly intervening on one side and by implication against the other in a matter of active and close-fought controversy in Northern Ireland, a part of the UK. His statement, that the issue was a matter for the Northern Ireland Assembly, was correct, but was he then entitled to make such an intervention, or to predict, as he did, ‘I am confident that, like other Western European countries, they will make that decision in due course’? I do not think so.

I know my view is an eccentric and lonely one in modern British politics. That's where I often find myself now. I can detect no sign that the Foreign Office has called in the Irish Ambassador to the Court of St James’s and suggested that such matters would be better handled in a more discreet way. But that just reminds me of Norman Tebbit’s old jibe that FO’s job in government was to represent the interests of foreign countries to Britain, rather than to represent the interest of Britain to foreign countries.

And so I now formally set out Hitchens’s second rule of politics (the first being that 'All politically significant statistics are fiddled'). I've implied it in many articles about Russia and the USA, but not stated it explicitly before. My second rule of politics is that 'All actions in foreign policy should be judged (by us and by our rivals and neighbours) by how we would view them if the positions were reversed.'