Incidentally, Kitty O'Doherty can be… a little unreliable and self-serving at times - here's her pension statement, for instance in which she refers to Fionán Lynch as having been 'boozed' and herself having been sent in his place to Tralee with mobilisation orders, and in comparison here's Fionán Lynch's own witness statement to the Bureau, which shows him as more importantly occupied.
But I'm sure her saying her sister wrote that song was honest.