Flying within Ireland is a breeze. You check-in online, go straight to the domestic lane at security in Dublin where there is never a queue and be at the gate 30 mins prior to departure. If you're arriving in Dublin airport, you can take Dublin bus to the city centre via the port tunnel in 20 mins.
ALLMCC, you are right, Ryanair could probably jump on this with massive publicity, saving the day etc.
I do remember that Aer Arann tried the route before, but that was when there was a good rail connection between Dublin and Belfast. They really have a more suitable a/c for such a short route but Ryanair have more balls..
Apparently, the Cork-Dublin route is one of Ryanair's busiest routes. It is also one of their shortest routes though the distance between Belfast and Dublin is a lot shorter.
Now who would bother taking the train between Belfast & Dublin? It could take a year before that viaduct is repaired and certified for use again.
Thankfully, at least we have the motorway now.