It's a stoater!

The Irish border is more than a three-pipe problem.  The Independent explains:

The reason why the Irish border issue hasn’t been sorted out more than a year after the Brexit referendum is that it cannot logically be the same as it is now – frictionless and seamless. When the UK leaves the EU customs union, with or without transition arrangements, some mechanism will be necessary to certify origins, to ensure that goods imported into the UK cannot travel into the European Union, ie Ireland, without some notification of their origin and whether they conform to EU rules and have paid EU duties, and vice versa. Otherwise the EU’s common tariff barrier and [with] the rest of the world cannot work. Modern technology and licences granted to trusted companies can help assist this, but the fact remains that some fresh bureaucracy, even if mostly digital in form, will be required, and human beings will be needed to police it.

Even if the customs union problem could be settled with countless ANPRs (automatic number plate recognition cameras) and CCTV posts, that still leaves the even more fraught issue of the free movement of people. There is nothing today to stop, say, a Lithuanian flying to Dublin, taking a train to Belfast and entering the UK.

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In other words, David Davis, Michel Barnier, the Irish cabinet and all the other clever people around the capitals of Europe have failed in their quest to make two plus two equal five. It is as if a team of mathematicians had promised to make two plus two equal five because that is what everyone agrees it should be – there is lots of goodwill behind the idea, it would make life a lot easier, and it would be much worse for peace in Ireland if two and two actually made four. Of course they could be locked in a room until the end of time and still not find a way to make two plus two equal five, because it can’t, and no amount of wrangling will make it happen.

The only solution (and it is essentially a non-solution) is for everyone to ignore the problem.  Maintain the status quo on the border and simply accept that there may be leakage of goods into and out of the EU Customs Union and of a modest amount of uncontrolled emigration from the EU into Northern Ireland (and thence into the UK)..  Would that be so bad?