The entrance, once accessible through an opening in the northern bank of the rath, was blocked up at some point in the past, and the ground now gives nothing away.

A rath is a roughly circular enclosure, typically defined by an earthen bank and ditch, built during the early medieval period in Ireland as a farmstead or place of habitation. Souterrains, from the French for "underground passage", are stone-lined underground chambers or tunnels associated with these settlements, thought to have served as places of refuge, cool storage for food, or both. The one at Tawnylough is known only through local tradition, which describes it as a cave-like space reached from inside the rath via that now-sealed northern opening. Whether the blocking was deliberate, precautionary, or simply the result of gradual neglect is not recorded. What survives is the memory of the thing rather than the thing itself, preserved in the kind of oral knowledge that tends to outlast the physical evidence by generations.