Fergie, I asked on another thread if the Old Triangle was originally a flogging triangle : they were once used in British prisons.If there was also a triangle at Kilmainham Jail , the suggestion that the purpose had something to do with the transportation of goods along the Royal Canal doesn't stand up . The song Moreton Bay mentions triangles btw: "Captain Logan had us mangled all on the triangles of Moreton Bay. "
Warning :
Another Mayomick conspiracy theory /fake folk possibility :
My very tentative suggestion if it was a flogging triangle : The Mountjoy prisoner who wrote Auld Triangle song had once heard the song Moreton Bay .