At its widest it spreads about five metres across, sinking less than half a metre below the surrounding surface. That modest dip in the earth is, in all likelihood, the ceiling of something that once ran underground.

Souterrains are stone-lined, corbelled or roofed passages built beneath early medieval Irish settlements, most commonly associated with ringforts, the circular enclosed farmsteads that dot the Irish countryside by the tens of thousands. Their precise function is still debated, but storage of perishables and refuge during raids are the most widely accepted explanations. When a souterrain collapses inward, as this one appears to have done, the roof stones drop and the chambers compress, leaving the telltale sunken line in the turf above. Bowman, writing in 1934, recorded a depression in the south-west of the same site measuring roughly five yards by four, and about two feet deep, which he identified as indicating a souterrain. The modern survey placed a second, larger depression extending towards the west, suggesting the structure may have had multiple chambers, as souterrains often did. The two observations together sketch the outline of a passage system that has been slowly folding back into the ground for centuries.