A cashel is a stone-walled ringfort, the kind of enclosed settlement that farmers and local chieftains built throughout early medieval Ireland, and at Castleroyan the original structure has been so thoroughly entangled with later field systems that unpicking what is ancient and what is relatively recent becomes a real puzzle. Drystone walls both encircle and radiate outward from the cashel, overlaying its fabric and blurring the boundary between the early medieval enclosure and the post-medieval agricultural landscape that grew up around it.

The cashel itself is an oval, measuring roughly 37 metres east to west and 28 metres north to south, defined by a stone-faced scarp that still stands up to about 1.45 metres high on the west-southwest side. Where the stonework is visible at all, it appears in rough courses of large stones at the southeast and west, but much of the scarp is either sod-covered or has collapsed into a disordered spread of rubble. Beyond the main enclosure, traces of an outer wall or scarp survive to the northeast and west, with a fosse, a shallow defensive ditch, of around three to four metres wide between the two, though again later interference makes it hard to be certain how much of this is original. Built into the outer slope of the scarp on the north side is a semi-circular structure, likely a limekiln, a small industrial furnace used to burn limestone into lime for fertiliser or mortar, suggesting the site remained economically active into the post-medieval period. A derelict farmstead from the eighteenth or nineteenth century lies just twenty metres to the northwest, associated with small rectangular fields laid out in characteristic parallel stripes, a common pattern of land management from that era.

Inside the enclosure, the ground is level but uneven underfoot. In the southeast quadrant, an opening at ground level leads into a souterrain passage, an underground stone-lined tunnel typically associated with early medieval ringforts and used for storage or refuge. In the western half of the interior, a shallow oval depression, defined by a stony rim and accompanied by a low linear rise extending five metres to the east, may also be connected to the souterrain system below. The site sits in open pasture, looking out over the Trimoge valley and across the grassland to the south, and it wears its layered history in a way that rewards a slow and careful look rather than a quick one.