The text given is [barring some Englishing of the Scots] that quoted in Thomas Davidson, "Rowan Tree and Red Thread" (Edinburgh: Oliver & Boyd, 1949), p. 18; source note is "Chambers' Journal (April 1939)".
Three centuries is a long time for a song to lie underground, and I have no doubt that the poem is a modern thing inspired by Geillis Duncan's song. I don't see how it would fit too well with the Lowe tune, however.