I'm pretty sure I saw this too today from Maghaberry. Heard the noise and looked up to see it heading West. Is that the direction you saw it heading?
Incidently, I saw an RAF one of these flying right over Maghaberry after taking off from Aldergrove last year. Nice and low, heading in the direction of England. It looked really large. I think it was last summer when the last big batch of troops were pulling out. It was clearly a C-17 as I got the binos out for a real good watch!