According to my Scots dictionary 'blaw' is 'oatmeal'.

According to Hamish Henderson, 'Besom Jimmy',or, 'Brechin Jimmy' was a man called Jimmy Henderson. Hamish learned the song from Davie Stewart and thought it was written around the end of the 19th century.

Jimmy MacBeath also sang the song and definitely travelled (and lived for 20 years) in Ireland. So too then, maybe' did Jimmy Henderson.
(See, 'Alias Macalias: writings on songs, folk and literature' by Hamish Henderson. 2nd ed. 1994. btw, a biography of HH is due out soon, by Tim Neat.