You need to write a diary explaining Ireland's structural dependence on the UK, which you bring up any time people think of doing something without Britain.

It's called "Northern Ireland". We've got an open border with the UK so we're constrained on border and immigration policy. We can't close the border as that would have unacceptable effects on people in border counties.

Thinking about it, I don't think that would have much effect on the enhanced cooperation things except on travel policy: you'd still need passport and border checks, and it's not as if you can simply walk from Ireland to France.

The UK leaving the EU might be problematic - we have to move stuff through them - but that's a matter of negotiating details.

I have no idea what the effect of a splintering UK would have. I doubt a federal unUK would have much effect, but full independence for lots of bits would be interesting, in the Chinese proverb sense.