there is no segregation of UK/Irish domestic and other incoming flights.
Not 100% correct.
Aer Arann has an exit lane (it doubles as an airport staff exit) by-passing immigration/security, exclusive to its domestic arrivals.
Also, there's nothing to stop a passenger flying UK-DUB-Irish Regional without being checked. Avail of online check-in for both flights, then there's no need to leave the A pier on arrival into DUB.