Most of the Shannon busses are either local busses or on the Limerick - Galway express bus. The Stewart busses would be airport only.
My point is that in Ireland, good public transport exists and in the US, public transport is sparse on the ground. A $400 return cost for an Uber would be painful after a cheap flight, considering Belfast to Dublin Airport on the bus is probably no more than $25 return. (far less than just the prerequisite 15% tip in the US on top of the Uber fare)
How many UK airports have flights to NY? 6?