Upvote:3
Of course they've thought of that, which is why flights within the CTA don't depart from the international transit zones at LHR, LGW, and MAN; you have to go through immigration to get from an international flight to a CTA flight.
Note also that DUB doesn't have an international airside transit zone; all DUB arrivals from outside the CTA go through immigration. (Edit: It's unclear to me whether arrivals from the UK may still go through immigration in Dublin.)