This is a cashel, a type of ringfort built from stone rather than earth and timber, and its roughly 30-metre diameter would once have enclosed a farmstead of the early medieval period. What makes it quietly arresting is less what survives than how completely ordinary agricultural life has overwritten it. The field wall that slices through the monument at the north-north-west and south-south-east is not vandalism so much as centuries of pragmatic farming, the stones of one era quietly absorbed into the infrastructure of another.

Cashels of this kind were the homesteads of farming families, typically dating from somewhere between the fifth and twelfth centuries, and they are distributed widely across the west of Ireland where stone was more readily available than timber for construction. The enclosing wall at Tomnahulla has collapsed to the point where, on the south-west side, it survives only as a scarp, a low earthen or stony rise in the ground rather than any recognisable walling. Associated with the enclosure are the remains of a number of houses, suggesting that this was once a functioning domestic site rather than a purely defensive one. The cashel and its internal structures are recorded together, though the buildings themselves are catalogued separately.