Did you ever get an answer to your Darby Kelly question? I take the point that the derivation (to a certain point back in time) stems from the lyric in the "Boiled Beef & Carrots" song and further that there is reference to one "Darby Kelly" in the Napoleonic ballad of the same name (in which the eponymous Kelly appears to have been an Irish drummer fighting in the Napoleonic Wars) but that begs the questions: (1) Who was Darby Kelly? (2) Was he a real person or merely a character in a song? and (3) Did he have a prominent Darby himself?!