A souterrain, the term for a drystone underground chamber or tunnel used in early medieval Ireland, typically for storage or refuge, this one had been built using seven ogham stones as structural material. Ogham is an early Irish script, generally dating to the fourth through seventh centuries, in which letters are represented by groups of notches and strokes cut along the edges of standing stones. Whoever constructed the souterrain had repurposed these inscribed pillars as lintels, packing stones, and even as a prop to support a cracked slab, with one stone placed upside down beneath a fractured larger one to keep it from collapsing. Bones and skulls, some reportedly human, were found inside the passage when it came to light again in 1838.

The discovery was made that year by workmen digging a field boundary across a slight rise in the demesne, a short distance west of the River Loe. The passage, which curved from a southern entrance toward the north-east, measured roughly 5.7 metres in accessible length and averaged 1.3 metres in height, narrowing from about 2.15 metres wide at the entrance to around one metre at its far end. The walls inclined inward slightly as they rose, reducing the width at roof level, a technique that allowed nine large slabs to span the passage from above. Six of those roofing slabs carried ogham inscriptions. Scholars including Atkinson and Romilly Allen visited and recorded the stones in the latter half of the nineteenth century, and R. A. S. Macalister later published readings of all seven inscriptions. Several of the texts reference a group or lineage name rendered as TOICACI or TOICAC, suggesting the stones may have commemorated members of a single kindred. One stone also bears a small encircled equal-armed cross on the face that was uppermost when it served as a lintel, indicating it had acquired a Christian mark at some point before or after being built into the structure. In 1940, the Office of Public Works removed all seven stones from the souterrain and re-erected them beside a nearby public road. The passage itself was filled in, and no surface trace now remains of the underground chamber that held them.