The song was first published in 1912, both words and music by [William] Percy French. Paddy Reilly was the driver who transported Percy from station to hotel years earlier. When Percy revisited Ballyjamesduff in 1912, PR had emigrated to the US. So he wrote the song in tribute, apparently PR was a great "character". Whether PR was his real name or not, I haven't been able to discover. Reilly is probably the most common name in Co. Cavan.