My understanding was that a galloglas was a big, heavily-armed shock troop.

In tbe main, CeCe Borjeson's explanation is the correct one. Barley was often carried as food by Gaelic fighters. If a man so provisioned were left to rot on open ground, or were too shallowly buried, the grain would often grow and indicate wbere his bones were. Hence, "The Wind that Shakes the Barley".