But not for long on the Scottish route: it will disappear in the autumn when Stranraer closes and Stena moves a few miles up the loch to Cairnryan.

To return to the OP, it depends from where you travel from in GB but, IME, flights are almost invariably cheaper unless you buy last minute or travel at a very busy time. Easyjet flies from several points in GB to Belfast (from 3 London airports, Newcastle, Edinburgh, Glasgow, Liverpool, Manchester and Bristol) and are usually the best bet.

Bear in mind that there is no free hold luggage allowance on Easyjet and you will have to factor that in the cost (but they do allow one standard size carry-on with no weight limit as long as you can carry it and lift it in the luggage bins).

The other airline to serve Belfast from a range of GB destinations (a greater one than Easyjet) is Flybe but they often are more expensive. Easyjet flies mostly to Belfast International (BFS), which is in fact in Aldergrove , some 20ish miles outside Belfast (regular bus into the city) whereas Flybe flies to Belfast City (BHD), which is in East Belfast itself. Like on Easyjet, there is no free hold luggage allowance on Flybe (except on expensive tickets) but their cabin luggage allowance has a weight restriction of 10kg.

bmi also fly to BHD but they are usually more expensive (although their pricing can be at times erratic and you can occasionally find flights at unexpectedly cheap prices)