"drive to NI and catch a ferry."
Of course, the implicit assumption there is that anyone going to NI is doing so to get to Britain. What about the mythical Romanian who just moves to Belfast and sets up a life there? Is that a failure of UK immigration policy? (Maybe it's not - I'm genuinely curious what people think). I'm not from NI, but it doesn't seem terribly polite of British politicians to suggest that people would go to NI just to get to the "real" UK (aka Britain), as though NI isn't a worthwhile destination on its own.
The options I've seen floated range from deputizing ROI immigration officers to enforce UK immigration policy, to imposing a hard land border between ROI and NI (I really, really, really hope we don't end up with that), to imposing document checks for travelers between Britain and NI, which seems to be suggesting that nobody from the EU would want to go to NI just to live in NI.
In my view, at least, all of these options are a bit shit.
(I use the term "Britain" here to refer to the island occupied by England, Wales, and Scotland, distinct from the UK, which is England, Wales, Scotland, and NI)