Don't know much about the relative size of the catchment areas - but I've often wondered about the possibility of routes from the West of Ireland (Shannon, Galway or Knock) to the West/South west of England or Wales - there was a piece on another thread about Air Wales having a Galway/Cardiff flight on their booking machine.
I know that Cork - Bristol and Plymouth have both worked before and wondered if Ryanair were thinking about having a go at Bristol, Cardiff or Bournemouth ex Shannon. I wonder if the rugby fraternity in Limerick would make us of the first two ?
Unfortunately for Shannon, loads from outside the UK and the main european cities are probably more dependent on the inbound passengers (i.e. European mainland tourists) than on the people of the mid-west deciding to take a cheapo trip to Germany.