As we noted months ago, that was the inevitable price of maintaining an open NI border with the Republic of Ireland. The bottom line:
In the end, the cost of keeping the politically sensitive border in Ireland frictionless is a weakened union between Britain and Northern Ireland.The author concludes polling suggests long term trends appear to favor an eventual merger of NI into the Republic, although current public opinion doesn’t yet favor it. As the Republic becomes less Roman Catholic in its laws and customs, and NI becomes more accustomed to free movement of people and goods across their border, merger may attain majority support.
What I haven’t seen described is how they plan to keep people from entering the U.K. from the EU after they’ve drifted across the border into NI. How, in short, to allow NI residents with U.K. citizenship free access to the U.K. while keeping others at arm’s length. Document control for all entrants - citizen and otherwise - seems inevitable.