Technically and legally it is a copyright work... music by Iain Maclachlan. There have been umpteen legal disputes over the setting of lyrics to the tune.   However, the main person who wrote an officially sanctioned version in 1963, David Silver - is now dead. He created legal disputes using the publishers Essex/Westminster Music. There are a few already existing lyrics, providing clearance is given, I don't really see why it is necessary to create yet another version. Anyway, it is better as an instrumental. Legally it is a copyright tune, not an old air. But Westminster Music seem to have given up trying to defend the tune. Even Mike Oldfield has a spurious credit and claim on his instrumental that Westminster Music have not defended. There are many versions on the internet with the tune stated to be trad., this is loss of control and over-familiarity of the tune rather than legal fact. It has spread so far and wide that the publishers can't stop the flood, and just pick up what they can. However, if you do not intend to make commercial use on recordings, and are just using it in context of a play - I can't see a law suit. Though you should give a courtesy credit to Maclachlan and anyone's work you use in any printed or physical version. Just be aware that my late father's own lyrics are a separate copyright work "in the years long gone by.... lovely dark island where memories stray' (C) 1963 Stewart Ross MCPS/PRS or "Away to the west... When I'm back once more upon the dark island" (C) 1963 David Silver Westminster Music or Alan Bell's lyrics or W. Gordon Smith's lyrics etc.