It's detailed as around an hour and ten minutes on www.dart.ie. (If you know what time you have to depart/arrive at use specific times to get the exact details)
It's a bit longer if you have to change at Connolly (Arrow then a Dart - how much longer again depends on the exact time) but this might be an option if the times suit better.
I've no personal knowledge of the trip, but from frequent use of the Dart the timetable is usually fairly good (at least compared to other public transport).