This is a complete shot in the dark, but I have McKean interests in Auchnacloy, Co. Tyrone, which is about 20 miles away from Moy.
I realise the name is spelt differently, but if you know anything of Irish spelling, you come to expect the unexpected. My great grandfather John was born in 1849, and joined the British Army about 1860, served in England, and later moved to Glasgow.
He had a younger brother called James, of whom I know absolutely nothing. He was born in 1851, so evidently not your James. However it wouldn't be inconceivable for the name to be handed down another generation would it?
You mention your James had the middle name Gayton. Could this be his mother's maiden name I wonder?
Best wishes, Mike