Jasus! Walt you are letting your overactive imagination run away with you.
Mountjay jail is situated on the south bank of the Royal Canal on the nortside of Dublin City. Many years ago (before there were cheap watches or electric clocks) it was customary to ring a bell to let the wardens in the various wings of a prison know the time of day (end of shift, feeding time, lights out etc. etc..). In Mountjoy prison they did not use a bell for this purpose, instead they used a large bar of metal that had been forged into the shape of a triangle (much cheaper than a cast bell) this instrument was suspended on a short chain in the exercise yard of the prison and it was beaten rapidly with a hammer at the oppropriate times.
Brendan Behan is reputed to have written this song, he was held prisoner in Mountjoy (universally referred to as the 'Joy in Dublin) Brendan was arrested for attempted murder of a Civic Guard (a policeman).
The song is evocative of the loneliness and tedium that prisoners experience, it is about life in prison it is not about the vagina, the genitals or womens' thighs. Get a grip on yourself Walt, you need to get out more.
Fergus