Barnard could have been his first name:

"The ballade of ane right noble victorius [and] myghty lord Barnard Stewart lord of Aubigny erle of Beaumont Roger and bonaffre consaloure and cham[ber]lane ordinare to the maist hee maist excelle[n]t [and] maist crystyn prince Loys king of France knyght of his ordoure capitane of the kepyng of his body co[n]queror of Naplis and vmquhile co[n]stable general of the same." - by William Dunbar, Walter Chepman and Androw Myllar, 1508.

Which would be a disaster for dating the song, as you'd have to look up everyone called Barnard, Bernard, Bernhardt etc.