I wrote my thoughts on this many times in previous episodes of this thread. I think that it's more important to keep the Northern Ireland border open than it is to prevent illegal immigrants crossing.
Most illegal immigrants that attempt to stay for long in the UK can be found by checks on housing, employment, bank accounts, spending records unless they use cash, records of who owns vehicles passing any number-plate recognition cameras and such.
Some illegal immigrants (more than the authorities admit) sneak in already on small boats, hidden in lorries, trains, cars, and by other means - so we need methods to detect them other than when they're crossing borders anyway.
The UK will continue to allow tourists from the EU to visit in any case - it's only if they stay too long or try to live or work here without permission that they'll need to be detained and expelled.