It's written by Brendan behan, an autobiographical account of his imprisonment in Dublin's much loathed Mountjoy Jail on terrorist offences, originally sentenced to death it was later comuted to a life sentenced then pardoned under a general amnesty. The song was used for his play The Quare Fellow. The triangle in question hung on the prison wall next to The Royal Canal, it signalled the days events from waking up, to slopping out, mealtimes and bed time, A very hated sound