I should probably have tested this already so I did a dirty little test by giving a Civ Nationalism with the World Builder and enabling respawns on every turn (in RiseAndFAll.py) as I ran autoplay. Sure enough, after a couple of turns the Green Island became... green!-It is unknown at this time whether the Celts will respawn after nationalism in Ireland (defined as its "normal" area) should they be defeated earlier. If anyone can confirm or deny this I would also be very appreciative.
Sure, this still needs to be verified in actual play. But the Normal Area (the respawn zone if you like) does indeed work as intended.