What makes the example at Gleninsheen quietly absorbing is its shape. Where the vast majority of Irish cashels are roughly circular, this one is rectangular, an uncommon form that already sets it apart before you begin to examine the masonry itself. It sits on a short flat terrace on the north-facing slopes of a karst ridge, tucked within what appears to be a multiperiod field system, the whole landscape layered with human activity across centuries.

The cashel measures approximately 33 metres on its SW-NE axis and 31 metres SE-NW, and its walls are substantial, between two and two and a half metres wide, rising to a maximum external height of just over two metres. They are built of vertically laid slabs and blocks, occasionally roughly dressed, and faced on both the exterior and interior surfaces. Tim Bowmer, writing in 2019, described it as a heavy-walled cashel, and that assessment is easy to credit. The NE corner is rounded while the NW remains angular, and a rubble-covered, splayed gap near the centre of the NE wall most likely marks the original entrance. Inside, two possible house sites have been identified, one near the centre and another in the SW corner, though later animal pens have been added across the interior and along the walls, the kind of pragmatic reuse that recurs across early medieval monuments throughout Ireland. The site does not stand alone: a lighter-walled rectangular cashel is appended to its NW side, another cashel of similar form sits 30 metres further to the NW, and a later circular enclosure lies roughly 85 metres to the SE. Together they suggest this part of the ridge was not simply occupied but organised, shaped and reshaped over a long span of time. The OS six-inch maps of 1842 and 1916 both mark the structure, indicating it was a visible feature of the landscape well before any formal archaeological interest caught up with it.