I have found a possible 18th century reference for "Barbara Allen" (Child #84). This is from NORTH PENNSYLVANIA MINSTRELSY, AS SUNG IN THE BACKWOOD SETTLEMENTS, HUNTING CABINS AND LUMBER CAMPS IN NORTHERN PENNSYLVANIA, 1840-1910, by Henry W. Shoemaker:

http://books.google.com/books?id=WiAwAAAAYAAJ&printsec=frontcover&dq=North+Pennsylvania+Minstrelsy&hl=en&sa=X&ei=VbCET4XwDejt0gG

Shoemaker says that this version of "Barbara Allen" was sung by Walter S. Chatham, 1777-1855. This means that it is possible that he learned this as a young man in the 18th century. Here is the link for the song:

http://books.google.com/books?id=WiAwAAAAYAAJ&pg=PA107&dq=Bonny+Barbara+Ellen-+North+Pennsylvania+Minstrelsy&hl=en&sa=X&ei=wK-ET

There are several other "Child Ballads" in this collection, including: "Charlie and Sallie" (a version of "Geordie"), "The Cruise in the Lowlands Low", "Katie Maury" (a version of "The Baffled Knight"), "Wooing and Death of John Randal" (a version of "Lord Randall"), "Lord Lovel", and "Lord Thomas".

The ballad "Charlie and Sallie" was learned by Dan Elliott "from his grandmother", but there are no reference dates given. This is also true for "Lord Thomas". The ballad, "Katie Maury" was sung in 1857. The version of "Lord Randall" was said to be "the version our earliest pioneers sang in Potter County,..."