Just found this in Kathleen Behan's (mother of Brendan and Dominic, among others) memoirs:

[1984:] Our Brendan spent so long in jail, he wrote a lot about it. He wrote a lovely song to go with his play 'The Quare Fellow', and called it The Old Triangle, after the metal triangle that used to be rung to call the prisoners to their tasks. (Kathleen Behan, Mother of all the Behans 108)