A cashel is a stone-walled enclosure, broadly of early medieval date, built to protect a farmstead or settlement. This one, known in Irish as Cathair Fhiodhnaigh, takes its name from the Irish word for woodland, and both the cashel and the townland it sits within preserve the memory of forests that once filled the valley below. When the antiquarian Thomas Johnson Westropp visited in 1908, he noted that large ash trees were still growing inside the walls, a quiet echo of that older, wooded landscape.

The structure itself is considerable. The enclosure measures roughly 29 metres east to west and 26 metres north to south, with walls up to 7 metres wide, standing 2.7 metres on the exterior at the north side. What makes it particularly interesting is the complexity visible within those walls. Three internal terraces are discernible, stepping up the interior face, and the technique closely resembles that seen at the great western stone forts of the Aran Islands. A depression near the top of the north wall, about 2.5 metres square and filled with collapsed stone, is likely the remains of an inter-mural chamber, a small room built within the thickness of the wall itself. The entrance, well-preserved and wider on the inner face than the outer, faces roughly east. Westropp, writing in 1901, recorded two flights of internal stairs, one to the north-north-west with six visible steps and one to the south-south-west with three or four, both now probably buried beneath rubble. He also noticed something telling in the masonry: some sections were carefully built of large flat stones laid in orderly courses, while others used large and small stones mixed together without any apparent system. That irregularity points strongly to different phases of construction or subsequent repair, the fort accumulated and patched across time rather than raised in a single campaign. The wider field system surrounding the cashel contains at least three other enclosures, including two heavily degraded cashels to the north-west and the cliff-edge fort known as Lismacsheedy about 200 metres to the south-east, suggesting this was once a densely organised landscape rather than a single isolated monument.