Long gone are the days of additional terrorist related security measures at Northern Ireland airports - its the same for everyone in the UK now
OK, would you mind explaining that to Ports unit of Special Branch. They seem to have missed that one...
For an example, there is still the requirement to inform SB if you are going from a non-designated airport to or from N.I. from somewhere in the common travel area. Just check the Terrorism Act 2000.
I've only operated into BHD and N.I for the last 15 years, so I don't know what I'm talking about...
The reason API is wanted for N.I is due to the change in the rules between N.I and RoI in regard to the common travel area that used to exist between them (politically it still exists.)
Since the UK e-borders stuff came in in 2009, it's meant that whilst you don't need to show your passport to come into N.I from RoI, you ARE required to show it if leaving N.I.
Now, if this sounds nuts, that's because it is.
The whole thing about domestic flights is a red herring. You shouldn't need API for an internal EU flight (the Canaries don't count for that though for example) but this is an anomaly where it's asked for by the UK Border agency.