Very appropriate that this thread should be revived after so long, as on 24th January 2020, it was the 100th anniversary of Percy French's death. This fact was mentioned by Cathy Jordan of "Dervish" on stage at Glasgow's Celtic Connections, during their concert on 19th Jan: she then sang another of his songs, Eileen Oge (or The Pride of Petravore). He was a prolific writer, and some of his songs may be well-known but people sing them without knowing who wrote them!
Percy French

My first impressions of "Mountains of Mourne" was that it was just another of those sentimental Irish songs, but it's full of humour, satire, irony and political comment all in one song!