Mountjoy was designed by the British military engineering officer, Captain Joshua
Jebb, Royal Engineers and opened in 1850, based on the design of London's Pentonville
Prison also designed by Jebb. Originally intended as the first stop for men sentenced to transportation, they would spend a period in separate confinement
before being transferred to Spike Island and transported from there to Van
A total of 46 prisoners (including one woman, Annie Walsh) were executed within the walls of the prison, prior to the abolition of capital punishment. Executions were done by hanging,
after which the bodies of the dead were taken down from the gallows and buried within the prison grounds in unmarked
The song is used to introduce the play, a story about the occurrences in a prison (in real life Mountjoy
Prison where Behan had once been lodged) the day a convict is set to be executed. The triangle in the title refers to the large metal triangle which was beaten daily in Mountjoy Prison to waken the inmates ("The Auld Triangle goes Jingle Jangle").
The triangle still hangs in the prison at the centre where the wings meet on a metal gate. It is no longer used, though the hammer to beat it is mounted beside it. In the original play by Brendan Behan, the song is written as the "old triangle" not "auld triangle".
The triangle was rung regularly to signify points in the prisons routine.
A second level of meaning is hinted at in the final verse in which the singer imagines himself dwelling in the women's prison. Another mourns the separation from
''his girl Sal". These hint at the internal erotic fantasies that prisoners use to separate themselves from the harsh prison environment. In this meaning the old triangle becomes the female pudenda and the Royal Canal the vagina. 
As with many Irish ballads, the lyrics have been changed with each passing cover. For example, the Dropkick
Murphys recording condenses the structure into a three-lyric section song with a chorus based on the last two lines of each stanza in the original.