Military history wise, Nigel, the 12th was an American Civil War unit - with lyrical references to "the South shall fade... and Southron rebel... etc. It seems to me that the already well known song about the original Clare's regiment of dragoons, a unit of Irish soldiers in French service since Ramillies (at least) was simply pinched and re-written. New York would have enough Irish inhabitants to be thrilled and aroused by the same tune. Clare's certainly pre-dates the 12th.... whatever the provenance of the original verses might be.