But as I have pointed out - flights aren't the main problem - it's the massive land border where people expect to pass to and fro unimpeded - and it would be very difficult to stop them!
They move freely now, why bother to change it?
Presumably people from outside the EU have to go through border controls anyway if they come to southern or Northern Ireland.
All you need to do is grant EU members the freedom to travel within the borders of NI and SI but not the UK mainland.
Anyone leaving Ireland without a NI passport has to go through customs anyway.
I don't see it as an issue and don't see why we can't amend the rules to suit NI, SI and their own interests.
It maybe NI might want to issue visas if people want to stay more than a certain time to stop total free movement and immigration.
But as I see it, since Brexit we can, ( within reason) make up our own rules as we go along.
We have to think " outside the box" for what works for us.