"Owen's Garden" was also supposed to be the name of a pub in Limerick where the 5th Royal Irish Lancers hung out. Edward Luce, who I believe was the first superintendant of the Custer Battlefield, wrote a biography of Myles Keogh called "Keogh, Comanche, and Custer," where he said that Keogh's father had been an officer in that outfit, and that Keogh had taught the tune to Custer. Unfortunately, Luce seems to have just made up a lot of stuff in that book, and there's no record of Keogh's father being in the military at all. Besides which, the 5th Royal Irish Lancers were deactivated during his lifetime. "Garry Owen" was always popular with cavalry units because it has a beat like galloping hooves, and Custer apparently knew the tune as far back as West Point, before he ever met Keogh.