A vertical section face, cut to a height of 3.5 metres by quarrying activity on the southern side of a rath, has exposed what appears to be part of a souterrain, an underground stone-built passage or chamber typically associated with early medieval ringforts and used for food storage, refuge, or both. It is not a dramatic discovery in the conventional sense; there are no grand chambers, no inscriptions. What you can see is a single horizontal roof lintel sitting just 0.65 metres below the modern ground surface, and beneath it a drystone wall standing 1.6 metres high, one stone wide, flanked by a band of brown earth that sets itself apart from the natural sandy gravelly soil around it. That difference in soil colour is itself a kind of evidence, a trace of the fill that once packed the space around the passage.

The souterrain sits in the south-western interior of a rath, the ringfort enclosure it was built to serve. Raths, the circular earthwork enclosures that dot the Irish countryside in their thousands, were the farmsteads of early medieval Ireland, typically occupied between roughly the sixth and twelfth centuries. Souterrains were a common feature within them, constructed from carefully laid drystone or occasionally rock-cut, and usually entered through a narrow opening from within the enclosed area. At Tawnamullagh, the quarrying that has damaged the southern portion of the rath has, somewhat paradoxically, made part of this underground structure visible for the first time. Three metres to the south-west of the exposed section face, in an area of the rath interior that has not been quarried, a single large slab protrudes from the sod-covered ground, possibly another roof lintel, suggesting the souterrain extends further than what the cut face currently shows.