There really weren't too many places with the same flight time as Birmingham it could have gone to. Most other likely candidates are already served from Cork. Maybe somewhere like Brest could have been an alternative, but as Brian points out, there is a well-established charter market to Jersey and they can flog a lot of the seats in bulk.
That said, I can't work out Aer Lingus' intentions with the Birmingham route. There's not a hell of a lot of point being on it if they aren't at least daily. Even then, they'd be better off putting on enough capacity to get the route to themselves. Maybe Ryanair going onto East Midlands means they don't see the same market there now?
Interesting news about RE looking at jets. It could allow them to operate longer routes from Cork, but presumably any proved route would become vulnerable to someone sticking an A320 or 737 on it.