"the 17th-century diarist Samuel Pepys recounts hearing the "Scotch song of Barbara Allen" sung by his favorite actress, Mrs. Knipp, and by Pepys' time it had probably already been a popular favorite for many years." ~~ programme note by The King's Noyse singing consort.
,..,
Probably so indeed; and why 'Scotch' if Pepys had not previously heard it in Scotch versions? And the diary entry suggests it was clearly a song familiar to him. Theories about its being a new song inspired by the Plague (and so, of course, likely to be cheerfully sung at social gatherings just afterwards!) do not seem entirely convincing to me.

~M~