Looking at the TT from Aug 26, from Portadown the Enterprise departures are indeed every two hours, at 08.30, 10.30, 12.30 etc.
I'll probably end up driving to Portadown and leaving the car at the P&R while I am in Dublin, for a few days, as I'll have my bike with me, to take on the train, and bikes are not permitted on the bus replacements...