Nacken Och Jungfrun (the water-sprite and the maiden) is an old Finnish-Swedish folk song. It has been collected in scores of different forms dating back as far as the 1780s. It sometimes goes under the title Näcken Bortför Jungfrun (the water-sprite kidnaps the maiden).
Gurt Lush’s arrangement is based on a recording of the song by the Finnish group Gjallarhorn, on their 2000 album Sjofn. You can find a rough translation of this version here. Some words have failed to translate, probably because the song is written in an old form of Swedish. Gurt Lush’s Scandinavian-in-residence, Vibeke Rowell, has kindly provided an alternative translation. Vibeke also informs us that the stringed instrument played throughout the Gjallarhorn recording is a Hardanger fiddle, which you can read about on Wikipedia.
There are other recordings of the song by other artists, but sadly not available on YouTube. However, if you go to Apple's iTunes store and search for "Nacken Och Jungfrun" you’ll be able to listen to fairly lengthy samples of versions by the Swedish group Svanevit and by duo Anna Emilsson & Jacob Lindberg. Both of these are quite different from Gjallarhorn’s version.