Rex,Charles Wolfe refers to 'Real Old Mountain Dew' in 'Kentucky Country' (Uni Press of Kentucky p121):
As Norm Cohen has shown, 'Mountain Dew' was first codified in its present form in 1920 by Ashville folk song collector and singer Bascom Lamar Lunsford, and it probably is related to an even earlier Irish song, 'The Real Old Mountain Dew'.He gives no specific reference for Cohen's comments.
Stewie.