This is a double lock and is beside the rear of Mountjoy Jail.
"A hungry feeling, came o'er me stealing
And the mice they were squealing in my prison cell
And that auld triangle, went jingle jangle
All along the banks of the Royal Canal."
This is from a play "The Quare Fella" by Brendan Behan.
Wikipedia article:
http://en.wikipedia.org/wiki/Brendan_BehanCategory:
prison canal lock