To clarify the Dublin airport situation...
Pier A arrivals are mixed.
Pier B has separate corridors for "Domestic and UK passengers" and all others.
Pier C also has separate corridors for pax arrving from Ireland/UK and everywhere else.
It is only Pier A where they are mixed.
Still, this raises the question that if most airport facilities here were designed such that Irish-UK flights had separate "lanes" for arriving pax, why are some passengers now being checked?
I always remember never being looked at when flying between UK and Ireland.. there was not even an inspection counter! You just went straight through to baggage claim. What has changed all of a sudden?
Having a common travel area should imply that the inspections are done at first point of arrival into the common travel area, rather like the schengen situation.. the passport controller at Schiphol, or wherever, is admitting the passenger into all the Schengen states, not just the Netherlands, and there are no further border inspections.
Therefore, a UK border inspector admits a passenger into not just the UK but into Ireland as well and vice versa.
If they cannot manage the situation in that fashion without all this haphazzard nonsense, they should simply scrap the common travel area!