The site is a rath, the Irish term for a ringfort, a type of enclosed settlement built in their thousands across Ireland roughly between the sixth and tenth centuries. Most were the homes of farming families of moderate means, their circular earthworks serving as much as a marker of status as a practical defence for livestock. At Treankeel, centuries of agriculture have done their slow work on the original structure, and field fences now run straight through what were once deliberate boundaries, making it a site where the past and the working landscape have become genuinely difficult to separate.

The rath sits on a south-east-facing slope of a ridge, on a stony terrace above the Trimoge River some 175 metres to the east. Its broadly oval interior measures roughly 32.8 metres north to south and 24.5 metres east to west, defined by a degraded stony scarp that stands only about half a metre high in places, merging at its north-eastern and southern edges with the natural fall of the ground. Outside the scarp, the remnants of a fosse, a defensive ditch, survive, about two metres wide and thirty centimetres deep at the south, with traces of an outer bank beyond it. A ramp-like stony slope on the east-south-east side of the enclosure may be where the original entrance once stood. Inside, the ground slopes and is uneven underfoot. Most intriguingly, a linear depression in the western half marks the presence of a souterrain, an underground stone-built passage typically used in early medieval Ireland for storage or as a place of refuge. Alongside it, a slightly raised sub-circular stony area of about six metres in diameter abuts the souterrain depression at its north-eastern end; its function is not understood. A low internal bank, set two to three metres in from the scarp edge, may be a later addition, perhaps a field fence of more recent origin rather than part of the original design.