I know nothing at all about that I'm afraid to say...I have literally, near On zero knowledge of that side....which army would that have been?
Could be either, if as the death cert suggests he was born 1885ish he could have joined the British Army and that is where he trained to be engine fitter and might explain why we can't find him in the census, but it could have been the Irish Army after 1922. Did you ever get his daughter's marriage cert, that would have his occupation at the time she married.
Do you know when his wife died or her age at the time?