The depression measures roughly seven and a half metres in length, a metre wide, and less than a metre deep, and several flat sandstone blocks still lie along it, resembling the capstones of a covered passage. What sits here is the partially visible remains of a souterrain, an underground stone-lined tunnel or chamber built during the early medieval period, typically beneath or beside a ringfort. Souterrains were used for storage, refuge, or both, and their roofing stones, when disturbed or collapsed over centuries, often leave precisely this kind of telltale trace at the surface.

Ringforts, the circular enclosed settlements that dot the Irish countryside in their thousands, were the dominant form of rural habitation in Ireland roughly between the sixth and tenth centuries. The souterrain at Shanakill belongs to this broader pattern, tucked into the north-eastern quadrant of the associated ringfort. The flat sandstone blocks still visible at the surface suggest the tunnel was roofed in the corbelled or lintelled style common to Cork and Kerry souterrains, where large flat slabs were laid across the side walls to seal the passage below. Whether the passage itself remains intact underground is not recorded, and the depression as it stands is more shadow than structure, a long shallow curve where the ground has settled over something hollow.