I was expecting a bit more from this given Healy's notes below. However, most of the souterrains are now backfilled again since his excavations 122 years ago. You can just see the corbelling of one, the second as he describes it.
We had traipsed up the right-of-way lane just off the Johnstown to Freshford road. The rath is enormous, thoroughly overgrown (especially now in mid-June) and utterly compelling. The inner ditch is over 5 metres deep in places, the south-western arc being the most accessible. There is the remnants of an entrance here too. I can't guess the diameter, 50 metres could be close, but it may be much more. This would, in my estimation, have been a 'high-status' (dreadful term) habitation site. It's yet another of those places that could be marketed (yikes) to joe public as it really is impressive and even held a fascination for my not very interested companion.
The Rath of Borrismore.
I have lately put men to work at the Rath of Borrismore, within a mile of Johnstown, Co. Kilkenny, which was traditionally said to have caves or underground passages, the covering flag of the roof of one being barely visible. I have found and fully cleared out three splendid chambers: first chamber is 10 ft. 6 in. x 6 ft. wide, and 6 ft. 2in. high; second chamber is 11 ft. long, and varies in width from 5 ft. 1 in. to 5 ft. 10in. , height of roof 6 ft. 2 in. to 3 ft. The doors are about 2 ft. 6 in. high, and 16 in. wide. The one from first room to second room is of inclined jambs: the others mostly the one width above and below. The third chamber is 11 ft. 4 in. x 7 ft., and has a door on its north side, but there does not appear to be any chamber or passage to which it gave entrance. They must have intended to construct others at a future time. The chambers follow each other in a direct line, and the opening was in the centre of the rath. They are built of limestone and gritstone, but no indication of mortar of any kind being used. Within a hundred yards of the rath are two quarries, one of limestone, and the other of a whitish gritstone, such as is used in the building of these chambers. I will publish a full account and dimensions of them shortly. They were all firmly packed with sand and stones as if to effectively close them from being haunts of robbers. About eighty years ago, Kilkenny men when digging for gold, broke in the roof of the third chamber. Its floor is 14 feet below the surface, and I have made a rude staircase for visitors to ascend and descend who would not care to travel on hands and feet through the doorways. – W. Healy, P.P., Hon. Provincial Secretary for Leinster.
The Journal of the Royal Society of Antiquaries of Ireland, Ser. 5, Vol. 1, pt. 2, p. 490, 1891