RJ100
That's probably because people don't wish to get off at Shannon on the outbound leg having just settled down for the flight. Anyway, why don't the immigration people at Shannon board the aircraft and clear the passengers that way - can't be that difficult surely, a lot easier for the premium paying passenger, than having to disembark taking their hand luggage with them as well.
Apologies for posting something that relates to Mainline here, but was responding to the above post.
FF