Catsphiddle/Leadfingers/Micca: according to the Motor Hackney Carriages By-Laws, a taxi from Belfast International to Portaferry costs about 50 Pounds (roughly 45 miles, 1.1 Pounds per mile, first 1.5 miles cost 2.5 Pounds ==> 2.5 Pounds + 43.5 * 1.1 = 2.5 + 47.3 = 49.8 Pounds).
Same price for the way back ==> 100 Pounds.

Might it be cheaper to rent a car? (No idea of the prices)
Or are you three non-drivers like me?

Just an idea...