We just returned from 3 days in Dublin which was very busy and worthwhile. We took Rick's advice and booked the AirCoach online to transfer from downtown Dublin to the airport. The fare was very inexpensive (14E per person)and the bus went non-stop to the airport. Only problem is that you need to book a specific time and when we booked a 9:15 AM bus it passed us because it was full. Online it says you must book a time so how can the bus be filled? The next bus was 9:30 and who knew if it would be full as well. There is no way to contact anyone at the company. We decided to share a taxi with another couple so as not to be too late to the airport. Of course, the fare for the bus was non-refundable. We have sent our complaint to AirCoach and are awaiting a response. But travelers should be aware of this problem. You can take a city bus which requires no reservation but it makes multiple stops and takes more that 45 minutes to the airport vs. 30 minutes.
Leave plenty of time to get to Dublin airport if you are flying back to the US because you need to go through customs there!