Tucked into the south-western quadrant of the hillfort's middle rampart is a cashel, a type of early medieval stone-walled enclosure, that was built not beside the older structure but directly on top of it. The south-western perimeter of the cashel sits on the hillfort's own rampart, and the builders appear to have gone further still, partly dismantling that rampart to use its stones as both a foundation platform and a ready supply of building material. It is a quiet kind of cannibalism, one monument consuming another across a gap of centuries.

The cashel is a subcircular enclosure roughly 30.5 metres in external diameter. Its wall, averaging around 2.25 metres wide and 1.9 metres high, is built in a style typical of Irish stone ringforts: an inner and outer facing of limestone boulders with a rubble-core fill between them, and a slight outward lean, or batter, of half a metre on the exterior face. The single entrance, just a metre wide, faces uphill to the north-east. Inside, the ground is roughly level, though bedrock pushes through the surface in several places, and a substantial rock outcrop sits in the western interior near the entrance gap. Excavations carried out in 1992 investigated how the cashel wall related to the hillfort rampart beneath it, and while dating evidence was sparse, the findings were consistent with an early medieval date, placing the cashel's construction broadly in the period from the fifth to the twelfth century. A second cashel has also been identified on the north-western perimeter of the hillfort's outer rampart, suggesting that the hillfort was repurposed in a more systematic way than a single opportunistic build might imply.

Mooghaun hillfort is large enough that visiting it involves a genuine walk through mature woodland, and the cashel sits within a landscape that rewards careful attention rather than a quick pass. The bedrock breaking through the floor of the enclosure and the visible coursing of the limestone walls give the site a legibility that excavation reports alone cannot quite convey.