The rath at Treanfohanaun sits on a natural spur partway down a north-west-facing ridge slope, with the ground dropping sharply away into a narrow gully to one side and opening onto the wet valley of the Gweestion River below. A neighbouring rath sits only 40 metres upslope, on the actual ridge summit, looking directly down onto this one, while two further examples lie within a few hundred metres to the north-west and south-east. Whatever arrangement of farmsteads, family groups, or territorial boundaries these structures once represented, their clustering here was deliberate.

A rath is a raised, roughly circular enclosure, typically constructed during the early medieval period in Ireland, somewhere between the fifth and twelfth centuries, and used as a defended farmstead. This example takes the form of an oval platform, measuring about 27 metres east-north-east to west-south-west and nearly 33 metres north-north-west to south-south-east, defined by a scarp rather than a built-up bank. The scarp is at its most imposing on the western side, where it reaches 3.7 metres in height, partly because it cuts into and works with the natural steepness of the ridge slope. On the southern and north-north-eastern sides, a narrow terrace, just over a metre wide, runs between the base of the scarp and the ridge itself; cattle now use this gap as a path. A possible original entrance at the east-south-east is visible as a low, slumped area in the scarp roughly two metres wide, though a later field fence now blocks it. Within the enclosure, a roughly square raised platform occupying much of the northern half of the interior adds another layer of complexity. It stands nearly a metre high on its western edge, where the ground falls away slightly, but fades almost imperceptibly into the surrounding interior surface on the eastern side. Blackthorn scrub has swallowed the northern and north-eastern portions of this inner feature, making it difficult to read fully. The same scrub, along with gorse, has colonised much of the rath's perimeter and pushed into the north-east quadrant of the interior, giving the whole structure a quietly overgrown, half-legible quality that the surrounding grazed fields do nothing to soften.