The UK has got to bite the bullet and reintroduce border controls in Northern Ireland then. Frankly I can't see why Ireland should get special treatment. Yes, they were part of the UK for a long while, but are not now. Treat them as every other country in the EU and scrap CTA. It's a good opportunity to see what works and what doesn't for when/if Scotland also gains independence and rejoins the EU as that will need border controls too.