It's primarily used as a feeder for Aer Lingus transatlantic now as well as a few other connections out of Dublin (e.g. Etihad). Because of that, they're able to charge pretty high prices for the legs.
It can probably get by on the numbers doing that until someone cops on that there are maybe 40-60 people making that connection in Dublin every day, along with a similar number connecting in Heathrow and maybe 20 in Amsterdam, along with 30-40 people making the overland trip to Shannon, which overall would add up to a decent load on a 757 transatlantic direct out of Cork.