We have found the best route for us is between Boston and Dublin. We have had good luck on Aer Lingus. We like the outbound flight that departs Boston at 9:30pm and arrives Dublin at 8:30am. On the return, we choose the flight that leaves Dublin at about 11:30am and arrives in Boston about 2pm. We also have done Boston to Dublin and Shannon back to Boston. Either way, the cost is reasonable; I highly recommend either route. A final thought: we don’t do car rentals, opting to travel by bus instead. Bus Eireann goes in and out of Dublin often. The cost for this is reasonable too; every bus has excellent wifi, and in Monaghan you can easily walk from the bus station to The Diamond. You can get a bus from Dublin Airport or from the Busaras in Dublin proper.