I like the Cork-Dublin train service, but it's not a good option for travelling to Dublin airport. One of the main reasons for this is the decision by Dublin Bus to turn the 747 into a leisurely sightseeing tour of Dublin instead of the express rail-airport link it used to be. It takes around an hour to go from Heuston to the airport.
I've used the Cork-Dublin airport Aircoach twice and it passed Heuston around 2:45 after leaving Cork, which is around the same time most of the train services take to get there, and arrived in Dublin airport around 3:20 after leaving Cork.
Hopefully faster trains and a rethink from Dublin Bus on the 747 will make rail a more attractive option.