I am no authority on the origin of songs but my thoughts are that the morning star is indeed the sun(it's a star after all)which suggests that there may be some pagan origin to the song.
I've also heard that it comes from the Appalachian mountains and was often sung at funerals.
Young Tradition did a brilliant version as previously mentioned which was recorded live at Cecil Sharp House, London at, I believe, their farewell gig.
In any case a wonderful and very moving song.