From within the UK? Well, I suppose the bus from Bangor will only cost you a couple of quid, but I suspect you mean from the other side of the Irish Sea.

There's a little-known train fare called SailRail, which includes both your rail and ferry ticket to Belfast. From Glasgow, it's £27 one way - so slightly cheaper than the ferry fare you've found!

seat61.com has an excellent guide to these fares. Other options include going via the ferry from Holyhead and then the train up from Dublin. Whichever station you start from in Britain, it won't cost more than £98rtn.

I've had a quick look at easyJet fares, and towards the end of June, prices to Belfast are similar leaving from London, so you might want to look around and see if there's a departure airport which fits in better with your itinerary. Remember that low-cost airlines like easyJet charge for extras such as paying by credit card, or hold baggage. They're also quite strict with weight and size restrictions for carry-on bags.