Surely the problem is stopping unauthorised people getting from the passenger area airside to the ramp rather than stopping people getting airside. I'd be a little disturbed if an passenger with a legitimate boarding card was wandering around the ramp.
Incidentally, I have noticed one potential immigration issue at Dublin. Certain flights from outside the common travel area with the UK land at pier A (for example Ryanair from Beauvais or Charleroi). With the cooperation of an EU citizen living in Ireland, someone could use this fact to bypass immigration.
Basically, what the person would do is book a flight into Ireland from one of these origins. The local resident would book an internal Irish flight on Aer Arann, check-in and proceed to Pier A. At Pier A, they would hand the boarding card to the person who has arrived and not cleared immigration. The local resident could then come back out of Pier A showing a legitimate passport while the person who hasn't cleared immigration could board the flight to Knock/Kerry/Sligo etc where their ID wouldn't be checked (Also, Aer Arann (legitimately) never check ID while boarding internal flights).
I doubt anyone's ever tried it, but it has the potential to be used.