The problem isn't Schengen: the problem is freedom of movement. These are two different things, even if people confuse them all the time. If there's no freedom

The problem isn't Schengen: the problem is freedom of movement. These are two different things, even if people confuse them all the time. If there's no freedom of movement between the UK and the EU, then the Common Travel Area is essentially dead, regardless of whether or not the UK and Ireland are in Schengen. This has massive knock-on effects on NI: the Good Friday Agreement is predicated on there being no hard border between the North and South.