A cashel is a stone-walled ringfort, typically early medieval in origin, and this one, known as Cahercullaun or Cathair Coileáin, retains enough of its massive wall to hint at its former scale: earlier observers recorded it standing up to twelve feet high and nine feet thick, though today the debris barely reaches a metre. What makes the site genuinely unusual is not the cashel itself but what was built into and around it at some later, still unidentified, date.

The cashel, roughly 24.5 metres across internally, was incorporated into the north-east corner of a large rectangular enclosure measuring 73.5 metres east to west and 44 metres north to south. The south-west section of the cashel wall was replaced or overlaid by what appear to be the ruins of a gatehouse tower, now two substantial mounds of collapsed stone standing nearly three metres high and 1.7 metres apart. When Curran visited in 1870, local people told him the gateway had once been arched over, with guard-chambers on either side. Chatterton, writing in 1839, recorded that the tower walls bulged at the base and that steps were still visible on the inner face leading up to the wall top. A possible souterrain, the name for an underground stone-lined passage associated with early Irish settlements, may survive in the north-east of the interior, marked by a shallow hollow and a low aperture sealed by two overlapping slabs, though its precise character is uncertain. A line of five stones nearby, one still standing upright, has no agreed explanation. Local tradition, recorded by An Seabhac in 1939, attributes the larger enclosure to Coileán Ó Duinn, or Ó Duibhne, who supposedly used the bawn, a term for an enclosed yard associated with a fortified residence, to protect his livestock from wolves at night. John O'Donovan, the nineteenth-century scholar and place-names surveyor, considered this a plausible reading of the site's function. The military road running about 150 metres to the north, crossing the mountain saddle between Ballysitteragh and Gearhane to link the Owenmore and Milltown valleys, suggests the site once occupied a strategically watched piece of ground, though whether that influenced the later construction remains, like so much else here, an open question.