its an old irish folk song that is used in the US military today, it is one of my favorite songs of all time after I heard it during the credits of BHD...
any listen to the Joe Strummer and The Mesceralos version if you can...its very touching, especially when played with bag pipes at a funeral and such...
Also I was watching a sort of old Mail Call (the one with the marine corps golf tourny and auction) and they played it during that too...I was like OMGWTFBBQTHATSAWESOME!
but with so many BD fans here, that I did not know of, you guys might really like it as I do....