Simple tune by Franz Xaver Gruber to lyrics by Joseph Mohr.
While in Austria last year I accidentally found the church where Gruber was organist and choirmaster in later life in Hallein and the organ was playing which was nice. However, this is not where the carol was composed - that is a place called Oberndorf but I suspect the tune was heard many times in the church in Hallein.
Uses the simplest two stops on the organ - gedackt and gemshorn.