It measures roughly two and a half metres long, a metre and a half wide, and barely a quarter of a metre deep. What makes it worth pausing over is what appears at its south-eastern end: a possible lintel stone, suggesting that the hollow is not a natural feature but the collapsed or deliberately blocked entrance to a souterrain.

Souterrains are underground passages or chambers, typically constructed during the early medieval period in Ireland, often built using dry-stone walling and large capstones or lintels to form a roof. They are commonly found within or adjacent to raths, the circular earthwork enclosures that served as farmsteads for much of early medieval Irish society. Their precise function is still debated; they may have provided refuge, cool storage for dairy produce, or both. The rath at Rathmorrissy, recorded separately, forms the broader context for this feature, and the souterrain sits within its north-western quadrant, aligned on a north-west to south-east axis. The presence of even a single visible lintel at the south-eastern end is enough to indicate that something deliberate lies beneath the surface, even if the passage itself has long since been filled or has fallen in.