The words will be copyright, since Henderson only died a few years ago, but he wouldn't have wanted his copyright enforced - he preferred his songs to float off into tradition. The tune is a slight variant of "Scotland the Brave" (or "Brave Scotland") as first published around 1880, so that will certainly be public domain. Henderson would not have wanted any harmonization to become "standard" - make up your own.

In the early published versions of the song Henderson doesn't name the tune - simply describes it as "traditional". It was probably not very well known by name at the time, since Hanley's words for it (the ones everybody knows) date from a year later.