From Bear McCreary's blog about this song: Some of the most authentic folk music you will hear this season is in this episode, in the scenes with the wool waulking group, and the traveling songs the Scots sing. I worked with the writers and producers to help select material for these scenes, but the true authenticity comes from the waulking group, who are the real deal. I was amazed at the footage when it came back because it was so authentic.
I spoke with writer/producer Toni Graphia about the song, and she said “The wool waulking song that Claire sings with the ladies is an old traditional wool waulking song called ‘Mo Nighean Donn.’  It translates to ‘My brown-haired lass,’ a term which Jamie later uses an endearment for Claire. We chose that song not only because it was beautiful and we loved it, but we thought it might be a nice little ‘Easter egg’ for the fans.”12