Of course its ok to play the tune its been played since 1958 and formally copyrighted since 1962. But obviously only a fraction of uses ever gets through to the PRS or whichever country's society your inm who distribute royalties. Then umpteen performers just shove it down as trad. Legally Iain Maclachlan (deceased) wrote it. But then you have people like Mike Oldfield registering a separate claim on it, even though he had nothing whatsoever to do with the work, other than recording it.   

As for John Mackenzie talking about maudlin lyrics... set by various parties including my father - it's a lament!!! The tune was written as 'Dr. Mackay's Farewell to Creagorry' and also known as 'Dr. McInnes's Farewell to South Uist'.. purportedly first written in 1958 by Maclachan. It's not supposed to be a happy go lucky jig - lament by definition, maudlin by mood. A slow air used for the BBC TV programme 'Dark Island' in 1962. Though it's been played in every conceivable style and tempo.