A stone-filled hollow, roughly thirteen and a half metres long, runs north to south and presses up against the ringfort's inner bank at its northern end. At the southern end, moss-covered stones extend westward in a way that suggests a second passage, approximately twelve metres long, branching off at an angle. Two underground corridors, partially legible, partially guessed at, lying inside an already ancient enclosure.

Souterrains are underground stone-lined passages or chambers, typically associated with early medieval settlement in Ireland, roughly the sixth to twelfth centuries. They were built beneath or alongside the ringforts, those circular earthen or stone enclosures that housed farmsteads across the Irish countryside during the same period. Their precise purpose is still debated, though storage of perishable goods and temporary refuge during raids are the most frequently cited explanations. The souterrain at Grange sits within one such ringfort, and the relationship between the two is typical of how these subterranean features tend to appear: tucked inside a defined enclosure, connected to the life of the settlement above. What makes this example quietly interesting is the tentative second passage. The moss-covered stones at the southern end do not confirm it conclusively, they suggest it, which means there may be more underground here than the visible hollow implies.