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.
The chapel nestled behind the houses on the east side of Meath Street which appear on the left of the drawing.