Rathlin (nothing to do with the song, afaik) is an island off the northeast coast of Ireland and the site of the 1575 mass murder of 200 defenders and 400 others, the extended family of Somhairle Buí MacDonnell - women, babies, little children, old people, who had hidden on the island in its caves, etc. Francis Drake led the butcheries. The Earl of Essex later described how Somhairle, held on the shore 100 yards away to watch, was “like to run mad from sorrow”.