We've got a little geographical problem.
1. If the Irish Border remains open, as promised - the following won't apply but it makes a mockery of the "Controlled Borders" dogma where we seem to be asking Ireland to do our immigration policy for us - which will mean letting in any EU national that cares to enter Ireland (hop on the bus across our unmanned border meaning "controlled border - what controlled border?) but checking non-EU's as they enter Ireland. Which is what they do already.
2. This is the following, and will apply if the border becomes hard. There's a ROI virtual exclave within NI. I say virtual, there's no other way to get from the exclave to any other part of the ROI except by road through the UK, or a short swim.
Inside that exclave appear to be 20 or so dwellings - mainly farms, some isolated housing.
To get to their nearest small town Clones (which belongs to the ROI) - maybe to shop, take the kids to school, whatever - they may in future have to go through two borders out (customs and immigration) that is from ROI into UK and then from the UK into ROI and the same two back - and have no other option save for the swim or hiring a balloon.
Will those residents be given special status and waved through?
There are a few enclaves/exclaves withing Europe, Baarle Hertog is one but both BEL and NED are in Schengen/EU. There's an Italian piece in Switzerland - but both are in Schengen. There's also a piece of Germany inside Switzerland...ditto Schengen.
Here's the place;
https://www.openstreetmap.org/#map=12/54.1350/-7.2857