That’s one of my favorite scenes, too. The Brit who taught them “Garryowen” is a fictional character, but Capt. Myles Keogh was born in Ireland and may have been the real source of that song. He died with Custer at Little Big Horn.
You might be interested in the connection between “Garryowen,” the film “We Were Soldiers,” and a hero of 9/11, Rick Rescorla. Check out http://webcache.googleusercontent.com/search?hl=en&gbv=2&gs_sm=3&gs_upl=1560l9657l0l10140l22l22l1l10l11l0l250l1717l2.6.3l11l0&q=cache:H2dKf0SR69sJ:http://www.stolinsky.com/news/news/default.asp?PagePosition=258+stolinsky+garry+owen+rescorla&ct=clnk
When I saw “We Were Soldiers...” I was disappointed that the score didn’t have “Garryowen.” I thought that would’ve had a very dramatic effect. Several years ago, my wife and I were on a coach tour of the western national parks, and for me, the highlight was going to Little Bighorn and seeing where Custer had been buried before he was disinterred and reburied at West Point.