The melody of this song is derived from part of the Irish air, "Eamonn a chnuic", "Ned of the Hill", about the seventeenth-century "rapparee" Edmond Ryan of Tipperary. In the play, isn't it supposed to have been made by one of the characters, an amateur versifier? I think there's a deliberate allusion to "Romeo and Juliet" in the third verse. Finally, during the congratulations which follow a nervous young singer's performance of the piece, someone mentions that he sings as well as "Count John McCormack"; this supposedly in 1913, about twenty-five years (I think) before this Papal title was awarded.

PS O'Casey used the familiar air of "When you and I were young, Maggie" for the song "Nora" in another of his plays, set in Dublin during the Rising of 1916. For the moment, embarrassingly enough, the title slips my mind.