The words are of doubtful parentage and in many cases were re-written. However in the 'folk proccess' many of these have changed from those originally written down.It is thought that all the tunes were in fact traditional and Serge Hovey tracked them down (for example Auld Lang Syne's proper tune is different to the one usually sung).
Serege Hovey being a modern composer then orchestrated the 'proper' tunes which makes fascinating, but different, listening and I highly reccommend the series of albums that were recorded with Jean Redpath doing the vocals. If you get chance to hear one of Jean's lecture/concerts they are brilliant.
All the best
Steve