The tune was written in 1963 by Ian MacLachlan, and was to begin with called Dr. Mackay's farewell to Creagorry and Dr. McInnes's Farewell to South Uist at various times. It was not called The Dark Island until it was adopted as signature tune for the television serial of that name; after that, several sets of lyrics were written for it, in some cases by people who were under the misapprehension that it was traditional. This information is available in previous discussions of the song, which I would suggest people read before adding comments here.