This is a Gaelic song air, the cuckoo in the tree. The tune appears in other sources e.g. Angus Fraser’s manuscript. The song is said to have been composed by the poet Uilleam Ros.
The tune appears on p.36 of the manuscript tune book compiled on the Isle of Mull by the Maclean-Clephane sisters. The tune is written out on a double staff as if to be arranged for pedal harp, which they played, but the bass clef is blank - only the tune is given. The title is written as Cuachag nan Craobh, and also above the tune are the two tags “Ancient Harp air” and “Mr (?) MacLagan (?)”
The sisters were interested in collecting the remains of the old harp repertory. I don’t know the reason for them saying this was a harp tune.