Passports relate to HM Immigration Service. Their officers are not interested in travellers from ROI or NI.
However, HM Revenue and Customs are interested in ROI arrivals, but not NI arrivals.
Police Special Branch are interested in both, for counter-terrorism reasons, along with any other flight in which they decide to take an interest.
Hence the work of art that are the flexible corridors in T1 arrivals, carousel 7.