Willy O'Winsbury is a traditional piece from the 16th/17th century. the version you refer to was done by John Renbourne (former Pentangle ) and can be heard on the album Faro Anne.
He has arranged this peace in finger style, where the accompaning play has realy nothing to do with the melody of the song. It is medium difficult to play and can be accompanied by cords and a second voicing mostly comprising from single notes.