
I guess a lot depends on where you are starting out from.
Stranrear or Cairnryan to Belfast is about a 2 hour crossing or thereabouts and the ferry terminal is 10-15 minutes from the showgrounds.
The Holyhead route will bring you into Dun Laoghaire (Dublin). Belfast is about a 2 hr drive in favourable traffic conditions...3 hours if you hit Dublin at rush hour. The crossing takes about 90 mins. If you do come via this route, be sure to have a few Euros on you for the toll road (dont know how much it is for transits...a car is E1.80 from memory but the price increases with the size of the vehicle.