While I think there is a market for Shannon-USA, it would seem to be a market that is better served by USA-based aircraft. It's not really worth operating a long-haul base unless you have 4-6 aircraft there. The overheads for the few flights to Boston and New York must be significantly higher than the overheads for operating from Dublin. It would almost certainly be better to leave the US routes to the likes of Continental who can serve the route from their US-based hubs.
BTW This is also a reason why Aer Lingus would be nuts to consider TA from Cork or Belfast.