Tower, who mentioned a car? Making assumptions I'm afraid.
As someone who has epilepsy, driving is not an option. Therefore, public transport is my only option when arriving into NI in the middle of a work day.
So, yes, it CAN AND DOES take up to 3 hours to get from Aldergrove to Derry for me. Perhaps you should give it a go some time, and as I said, I'd prefer to use my local airport, rather than BFS, where I have had one too many poor experiences.