Octagonal is probably a bit of an exaggeration, the Meath Street Chapel was essentially a square building with splayed corners internally.
McCullough extracted the plan of the chapel from the 1838 O.S. map and his drawing illustrates the U-shaped gallery that ran around three sides.
I’m not aware of any actual illustrations of the building.
![](https://i0.wp.com/imagizer.imageshack.us/v2/640x480q90/822/i711.jpg?w=640)
The chapel nestled behind the houses on the east side of Meath Street which appear on the left of the drawing.