It is easy to miss, the kind of feature that registers only as a slight unevenness underfoot, yet the earthworks here preserve the outline of a rath, a type of ringfort typically formed by a single bank and ditch encircling a family farmstead. Thousands of these were built across Ireland between roughly the sixth and tenth centuries, and Tullahennel holds one of the quieter survivors.

The site takes the form of a raised sub-circular area sitting about 0.6 metres above the surrounding land, with internal measurements of roughly 29 metres north to south and 24 metres east to west. Along the southern and western arcs, an earthen bank remains visible, standing 1.2 metres high on the outer face, 0.9 metres on the inner, and spreading some 7 metres in width, figures that suggest a once substantial enclosure even if the full circuit has not survived intact. The classification as univallate simply means it appears to have had a single surrounding bank rather than the multiple concentric rings found at higher-status sites. The rath appears on both editions of the Ordnance Survey maps of the area, though by the time the later edition was produced a fieldbank had been driven across the site on a northeast to southwest alignment, cutting through what remained and complicating the picture for anyone trying to read the earthworks on the ground. That intrusion, routine in the context of centuries of agricultural reorganisation, is part of why the site looks fragmented today rather than forming the clean ring a plan drawing might suggest.