To perhaps illuminate the origins of this song, Click Here. This is an introduction to the tune from Liam O'Flynn, the piper who was a close friend of Willie Clancy's. Date of recording was late 1972, about six months before Willie died. As he says the words were known in Ireland but the tune was lost, and his source in Warsaw was an American girl whose parents or grandparents were Irish. Liam I think had formed Planxty with Andy Irvine, Donal Lunny, and Christy Moore by this time, so certainly Christy could have had a chance to learn this from Liam or Willie. Never heard his take or Hamish's so I can't say which one it resembles more.
From the liner notes of Seán 'ac Donnchadha's LP An Aill Báin: The White Rock (Claddagh CC9), liner notes by Seamus Ennis = "Dark is the Colour of My True Love's Hair. I had already heard this song sung by Robin Roberts of New York, to her own guitar accompaniment, but to another tune, when Willie Clancy of Miltown Malbay, County Clare, brought it home from a folk-festival in Warsaw in the 1950s, and played it on the Uilleann pipes for me. Many of us fell immediately in love with the tune and Seán made sure to learn the words of the song too. He tells me it is now one of his favourites when he is in the mood for this type of song."