It's a pity that by the time he was asked to contribute to the annotations on the Scots Musical Museum, Sharpe had lost the document mentioning a real Barbara Allan. (See Richie's posting at 11:40 on 26 April.)

With all due respect to Sharpe's scholarship, he also thought that the stanza mentioning ships supported his belief that the ballad originated near the Solway, as if there weren't ships to be seen all round the British coast (see the same posting by Richie), so perhaps his desire to find a local origin for the ballad overrode his better judgement.

On the subject of real Barbara Allan's, the 18th century Scottish artist David Allan, who produced a series of illustrations of Scottish songs in the 1790s (including one of "Barbara Allan" theatrically taking to her bed while her mother looks on), and who according to at least one anecdote was no mean singer of traditional songs himself, had a daughter called Barbara... but I imagine the name was most likely in the family and nothing to do with the ballad.
(See http://www.educationscotland.gov.uk/rsascottishart/imagedetails/thechildrenofdavidallan.asp)