A causeway seven metres wide at the eastern side of the smaller fort leads directly into the interior of its larger neighbour, suggesting that whoever built and used these structures thought of them not as separate places but as parts of a single arrangement. The smaller fort sits to the west, the larger to the east, and between them lies a fosse, a ditch cut into the earth, that simultaneously divides and links them.

Ringforts, known in Irish as raths when built primarily from earthworks, were the most common form of enclosed settlement in early medieval Ireland, typically serving as farmsteads for a single family or extended household. The Mortlestown example is oval rather than perfectly circular, measuring roughly 28 metres north to south and 34 metres east to west internally. It is enclosed by an earth and stone bank, a fosse outside that, and in places a further external bank beyond the fosse, though this outer bank is only traceable from the south around through the west to the north, where it disappears into the enclosing fosse of the larger adjacent fort at the northeast and southeast corners. The northern arc of the enclosing bank is the most substantial stretch that survives, rising to between 1.5 and 3.5 metres externally and accompanied by the widest and deepest section of the fosse. A causewayed entrance three metres wide on the western side gives access directly into the fort's interior. The whole sits on a low hillock in gently rolling Tipperary grassland, with open sightlines in every direction, a position that would have made good practical sense for anyone keeping livestock or watching the surrounding landscape.