My one experience with Aer Lingus (Newark to Dublin to Glasgow, and Liverpool to Dublin to JFK) was fine. The only quirk was that there was only one drinks service on the flight over; if you wanted wine (or even soda) with your dinner, you had to get it before they brought the dinner service, as they didn't bring the drinks cart down the aisle a second time. Also, their carry-on size and weight limits are bit tighter than a US carrier (the over the seat bag can only be 21 inches, not 22 inches). My bags were not weighed or measured, but I made sure to adhere to their requirements so there would be no trouble, just in case. Otherwise, it was like any other airline in coach, and I'd certainly fly them again.
Something else to know if you're returning to the US from Shannon; you go through US immigration and customs in Shannon, before you board your flight to Boston (or anywhere else in the US). Your arrival in Boston is thus like a domestic arrival. So, you don't need as much time to make your Boston connection as you would on a regular international to domestic connection, but you need to allow more time at Shannon to go through the "formalities."