Yesterday was a busy day! My son and I had an adventure, taking the StenaLine Superfast Ferry from Belfast to Cairnryan, on a daytrip which included entrance and unlimited rides at M&D's, a theme park just outside Glasgow.
We chose to take our car, which costs from £31 for an adult, including M&D's, or you can book a coach tour which starts at £28 per adult. There are benefits to both choices. With the coach you can relax and don't have to worry about directions, driving, or petrol costs. We chose to drive however as you can go at your own pace, stop off at services for snacks, or a quick peek at Ayr on the way past, plus you've your own privacy in your car. I still get car sick on buses, so driving was definitely the best option for us, plus I love driving places I've never been before, it all adds to the adventure!
http://www.stenaline.co.uk/ferries-to-britain/day-trips/belfast-cairnryan-mds-car
http://www.stenaline.co.uk/ferries-to-britain/day-trips/belfast-cairnryan-mds-coach
So, I'll talk you through our trip, and you can see how it would work for your own trip.
Check in at the Belfast docks is before 7am, so it's a very early start. You'll need a print out of your booking etc. but we didn't need passports, or any ID, although it recommends you have them with you just in case. You are given a lane to wait in, then you drive into the boat in turn. Once you park up you can head up into the main part of the boat.