I've taken the CityLink bus a few times and never had any problems. Although maybe I've been lucky; other people may have had different experiences.
4am from Kilbeggan - I guess that's the 2:15am from Galway? Always heavily booked, make sure you book in advance. May also be a good idea to phone them the previous day and impress on the office that the bus must stop in Kilbeggan!
Also, make sure it's an "Airport Express" that doesn't go through Dublin city centre.