These related songs have a complicated history that is a bit hard to sort out, but having read several threads on the subject, I believe the lyrics you have called THE DARK ISLAND were indeed written by Stewart Ross, but the lyrics in the DT contain some errors and are missing a verse. The correct version has been posted by the author's son, Alan Ross, here.
The lyrics you call DARK ISLAND 2 were written by David Silver (not "Silver Maclachlan").
Both lyrics were set to a pipe tune previously composed by Ian (or Iain) McLaughlan (or Maclachlan) – I don't know which spelling is correct – but Alan Ross says that Stewart Ross modified the tune a bit to fit his lyrics.