I haven't heard the Dubliners' version, but the original version was on Billy's 1974 album "Cop yer whack for this" - the title of which comes from an anecdote about his "lucky" uncle Freddie, featured on the LP. Another military reference. Great punch line too.

On the original, the soldier has "A brother in Partick with long curly hair" - i.e not "a brother in Glasgow". Partick is the area of Glasgow where Connolly grew up, so we can assume the brother to be Billy himself.