The Dark Island was a radio play in 6 half hour instalments and the tune (which is brilliant) seemed to linger in pub session hell for many years.Brain MacNeill plays the best version I've heard on 'Horses for Courses'where it is credited as composer I MacLachlan, publisher Westminster Music.
There is a Gaelic version of the words available on assorted 'Best Gaelic Songs' compilations
All the best
Steve