I’m fairly sure it was the Old IRA that were recognised as the legitimate army of the Irish Republic in April of 1921 by the First Dail. They would have fought in the War of Independence and the Civil War meaning he’d have been in what was seen as the Irish Army in 1922, which is why I looked at the 1922 census – no luck. I may have been mistaken in expecting to find him there but at least it's one resource crossed off the list.
Joseph is pretty unlikely to have been in World War one as he was only 14 at the outbreak. If he was Old IRA I can’t see him joining the British Army when so much was happening in his home city by the time he was older.
Militaryarchive.ie also have a Bureau of Military History 1913-1921 and he isn’t showing in any of their records either. They would have records covering Cumann na mBan, IRA, IRB, ICA, Volunteers, Sinn Fein and non-affiliated people, which is why I held out so much hope. They also have an IRA membership series.
I do know that some people who were entitled to their pension did not apply. Some people had the attitude that they had done a civic duty and didn't need the recognition of a pension or medal. While I think that's a very noble ideal it doesn't help nosy descendants like me.
I’m going to slog through the individual membership records and hope the search option just isn't comprehensive of the whole collection.
If anybody can suggest where else to look I’d really appreciate it.