Originally Posted by
pineapple stu
You noted that you can't leave the field of play without the ref's permission. This is wrong; you just can't come back on without permission. So Réiteóir's right.
The obvious corollary from this is that a player can play someone offside by stepping off the pitch, but as this involves a player staying well behind the rest of the defence and thereby causing the problem in the first place, it's not something which is ever likely to be taken up as a tactic. I'd imagine it'd be a bookable offence to deliberately try catch someone offside like that too.