Part of this stems from [Operation Gull][1], which is in place on a non-systematic basis at English airports as well as Northern Irish air- and seaports.

It is a measure to identify people intending to use the Common travel Area (a British-Irish semi-open border concept) as a loophole to enter the republic of Ireland, as border controls at the land border are limited to spot checks on the motorway and trains between Belfast and Dublin.


  [1]: http://www.lawcentreni.org/policy/policy-briefings/203.html