FRA could be a very good route alright.Such a service would give access to the main european hubs of all three global alliances, OW @ LHR, ST @ CDG & AMS, *A @FRA. Not bad.
PRG has fallen in popularity a bit, I think. It has gained a reputation for rough stags, etc. The slowing economy and fewer immigrants is probably not helping the flight either, the connection traffic CZ had on the route probably also helped. Looking across the board in Ireland and the UK flights to Eastern Europe seem to be dropped more quickly than other destinations.
As for the 5th airbus, I guess it depends how well or badly ORK is doing relative to both DUB and BFS and the opportunities that exist out of ORK. I would guess a Scandanavian destination and a Northern Italian destination might well work. BRU might also work, as might VIE. Is that enough to justify a 5th a/c? probably not. At best a few 'w' patterns on the UK routes might free-up capacity to add new routes.
Brian.