The short answer to your question Red is that we don't yet know. My guess is that it will depend on what arrangements the EU and UK come to post Brexit regarding Visa requirements - if any. I guess ROI/UK citizens could still move freely under the CTA but citizens of EU countries might need a visa if nothing else is agreed. If a sort of visa waiver deal is signed (similar to Thailand) then visitors will simply need their passports stamped and scanned at port of entry to UK. Not applicable to UK/ROI citizens moving within the age old CTA. Any non UK/ROI citizen found wandering around NI without a stamp in their passport would be in trouble - as happens in Thailand. In answer - I don't know what will happen.