Where is that? My experience has been that flying to Ireland to the UK presents no border control whatsoever, though from London to Dublin I do have to present a passport (as a non-EU citizen) since Dublin routes everyone through border control. I believe that has more to do with it being impractical to set up a domestic terminal.

Interestingly, an Irish or UK citizen born in the British Isles need only convince the immigration officer of their bona fides, even without a passport.