I am going over but we normally fly out Sat and back Sun from Glasgow and drive to Coleraine. Used to bike it each year but after a few successive wet trips we changed to flying. It's cheaper but if you're biking and the weather's good it's one of the best weekends you'll have. Not sure if you've been before but make sure you ride the coast road up from Belfast via Cushendun, Bushmills, etc. As you have realised, you need to get booked up with ferries and accommodation quickly. The Coleraine tourist office will find you somewhere if you're stuck (do a web search).
Hope you enjoy it. I can suggest some good viewpoints if you want.