One thing the Brexiters did not lie about was their emphatic assurance that the island of Ireland would experience, as Theresa May continues to insist, “no return to the borders of the past”. What they did not say was that there will be a whole new frontier – welcome to the borders of the future.

We are not going back to the way things used to be before the UK and Ireland joined the European Economic Community in 1973. We are going forward, accidentally and haphazardly, into a division of the island that could be more profound than it has ever been. At best, it will be a border between the UK and the European Union itself. But at worst, it could even be a border between a new Trumpian world order and a Europe struggling to hold on to a notion of transnational democracy. Yet nobody seems to be thinking much about these possibilities.

Enda Kenny says Ireland will oppose any post-Brexit 'hard border' Read more

Politics are in turmoil in both Irish jurisdictions. In the republic, Enda Kenny is being shooed towards the exit door because of the failure of his government to deal with an extraordinary scandal over the alleged smearing of a police whistleblower, Maurice McCabe. In Northern Ireland, there is an election brought about by the almost equally extraordinary scandal of a breathtakingly expensive renewable heating initiative.

Both issues are important. But neither is of the scale of what Brexit is threatening to do to Ireland. What is at stake is quite literally Ireland’s place in the world. An island that has been bedevilled by great uncertainties of belonging is being forced to think again about where it belongs in an even more uncertain geopolitical context.

Assurances that there would be no hard border between north and south when Northern Ireland is hauled out of the EU against its will were always lazy and reckless. At heart, they were based on an arrogant assumption by many Brexiters and some unionists that the republic is really still no more than an eccentric adjunct of the UK. It wasn’t necessary to think the thing through in detail because the Irish would quickly realise that their best bet was to follow Britain’s lead out of the EU and into the new Isles of the Blest that would emerge in the Atlantic. And even if they didn’t, the Irish authorities would obediently agree to operate British border controls at Irish ports and airports.

As Brexit moves from airy fantasy into messy reality, only the most deluded still think any of this. Ireland will remain in the EU, and because it is part of the EU, the Irish border will be an external EU border. Even if the implications of this fact for migration can be fudged, the border will be inescapably present. The new line from May is that it will be “as fluid and frictionless” as possible – not so much a UK border, perhaps, as a KY border. But no amount of verbal lubrication can ease the reality that the UK (and thus Northern Ireland) will be in an entirely different customs regime to the republic.

The need for a quick deal will suck the UK into the gravitational field of Trump’s assault on liberal democracy

So when the UK does its fabulous trade deals with Donald Trump’s US, what happens to the KY border? The Oxford-based economic historian Kevin O’Rourke has cited the simple example of one of the things that Trump would undoubtedly want in such a deal: duty-free access to the UK market for cheap, hormone-enhanced American beef. The EU, including the republic, will keep a 15% tariff on this beef. Without rigorous border controls, clever importers in, say, Newry, could bring in that US beef without paying duty and send it across to Dundalk and thence into the whole EU. And the same could go for cars or steel or anything else that Trump would love to boast about exporting to save American jobs. The EU’s own customs union would become a nonsense. Why on earth would it allow that to happen? It won’t – customs checks, with all the economic cost and all the psychological irritation, are inevitable.

But there’s something even larger at stake now. The consequences of Britain’s need to replace EU markets are not just economic. The tectonic plates that underlie the current political architecture are shifting. When reality bites and Britain realises that the EU is not going to give the UK back all the cake it has eaten, there will most probably be a ramping up of nationalist and anti-European rhetoric in England. And the need for a quick trade deal with the US will suck the UK as whole into the gravitational field of Trump’s reactionary assault on liberal democracy and transnational institutions. If that happens, the Irish border will become not just an economic, migration and political frontier. It will become an ideological boundary.

None of this is inevitable – and in truth none of us has a clue where the Trump escapade will lead – but it is a live possibility. We have to consider a grotesque absurdity: that the road between Newry and Dundalk or Lifford and Strabane leads from one geopolitical zone to another. On the one side there is the neo-nationalist world order; on the other the rather isolated edge of an embattled transnational EU. This would make the “borders of the past” seem like garden paths strewn with rose petals.

The Guardian view on the Northern Ireland election: vote to keep the border soft | Editorial Read more

Even in the most benign scenario, where post-Brexit Britain somehow escapes the clutches of a reactionary nationalism and does not fall into Trump’s orbit, Ireland has to deal with a profound question of belonging. It has long enjoyed the luxury of not having to choose between being part of the EU on the one hand and being closely intertwined – culturally and economically – with the Anglo-American world. But as these two spheres drift apart, Ireland risks being pulled asunder if it tries to stay with both. It will have to think of itself, politically and psychologically, as a more European country. Which would be all very well if part of the island were not being forced to define itself as much less European.

There is only one way to avoid this and that is, of course, for Northern Ireland to remain effectively within the EU as a special zone whose recent history justifies its status as an exceptional place – a part of the EU in the UK and a part of the UK in the EU. That, we will be told, is too great a stretch of the imagination. But it is not nearly so big a stretch as the crazy divisions that the Brexit zealots are imposing on a small island that was trying hard to escape from the legacy of the narrow nationalism they are embracing.