Well, as soon as you write it, and you have it down. Make sure you have dated evidence.. post a copy to yourself.. and keep it sealed. Register it with copyright agencies (if you have enough works to join), or assign some rights to a publisher. Keep any press clippings, recordings, interviews and anything to do with the songs, so you can prove its origins, One problem is when something becomes popular, it takes on a life of its own. We live in a viral media age and worldwide copyright is a control problem. It doesn't take long for a song to become taken by others to be traditional, particularly if there is no-one protecting it. It's all a catch 22, you want something to be popular and enter the public consciousness - but as soon as you do, it means other people may take it, intentionally or not. Anyway, this one belongs in its own thread..of copyright protection.   Dark Island as an instrumental, was a confusing one, due to the title being changed, and having been played for at least a few years before being physically recorded and used as the theme for the 1962 TV programme. Then all the hassles arose over competing parties writing lyrics to it afterwards.