Nottamun Town was indeed collected by Cecil Sharp - from the Ritchie family. It's copyrighted by Jean Ritchie because she made up the fifth verse. Jean Ritchie wrote to Roger McGuinn;

The song … was collected at the Hindman Settlement School in Knott County, KY by Cecil Sharp around 1917 from the singing of my sister Una who was a student there (Una was 4th in our family of 14; I'm 'the baby one,' and am 77 now).

If you will check in Sharp's book of Appalachian songs he collected, you will find the Ritchie version … as notated from the singing of Una and Sabrina Ritchie (Sabrina was our cousin).