The circular earthwork here, roughly 41 metres across from north to south, is a rath, the Irish term for a ringfort, one of the most common early medieval monument types in the country. These enclosures, typically dating from around the sixth to the tenth century, served as the fortified homesteads of farming families, their earthen banks and external ditches marking out both a working household and a social boundary. What makes this particular example worth attention is not just its preservation but the detail that survives in its fabric, and beneath it.

The bank that defines the enclosure varies noticeably in height depending on where you measure it. On the western side, the external face rises to around 2.2 metres; on the east, it is closer to 1.7 metres. Outside the bank runs a fosse, a ditch roughly three metres wide, now visible only as a shallow depression, its eastern portion cut across by a later farm track. A gap of about two metres in the bank to the south-east almost certainly marks the original entrance, though a field wall built at some later date now blocks it. Perhaps the most quietly remarkable feature lies just west of the interior's centre: a souterrain, detectable at the surface as a slight depression, with a large slab of stone protruding from its south-western end. Souterrains are underground stone-lined passages or chambers, commonly associated with ringforts and thought to have served for storage, shelter, or refuge. The slab at Tullanacorra is a visible reminder that the ground here holds more than it shows.

The rath sits on a low rise with open views, particularly westward and south-westward along a stream valley. Its interior is grassy and relatively open, ringed at a moderate distance by hawthorn trees. The bank has been worn through in several places by farm stock, including gaps to the north and east, and the enclosure remains within working agricultural land, so the earthwork exists in a lived-in, slightly roughed-up condition rather than as a tidy monument. The slight depression of the souterrain and the protruding slab to the west of centre are the details most worth seeking out once you are inside the bank.