Someone mentioned Kilmainham Jail in this regard. It was Mountjoy Jail that the song is about and Brendan Behan whilst most associated with this song never claimed to have written it. In fact, there is a recording of Brendan on Youtube expressly saying he did not write it. A granduncle of mine who served time with Brendan's father explained the Triangle thus. At the back of the 'Joy (Mountjoy Jail to you and I) runs the Royal Canal on which horses walking on towpaths pulled barges. The apparatus required to facilitate towing the barge parallel to the towpath had to be triangular, and the sound of these triangles jangling to the horses' movements represented the sound of freedom to the lags inside.

The old triangle went jingle jangle,
all along the banks of the Royal Canal

I knew of this explanation from childhood and never thought to question it but when I heard the Dinner Bell and Work Bell explanations I asked the late Brendain O Duill, actor, raconteur and singer and friend and admirer of Brendan Behan, he had exactly the same explanation. Whatever explanation, the song is a jewel and Behan always made sure the royalties when he sang it, went to the author.