The rath at Rathwilladoon is not the largest of its kind, measuring roughly 39.6 metres east to west and 38 metres north to south, but what makes it quietly unusual is the souterrain at its centre. A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period and associated with storage, refuge, or both. Finding one positioned at the heart of a rath rather than along its interior edge or beneath an adjacent structure is a detail worth pausing on.

The enclosure itself is defined by a bank and an external fosse, the fosse being a defensive ditch cut around the outside of the bank. For much of its circuit, the bank survives visibly from the north-north-west around to the south-south-east, though elsewhere the ground simply drops away as a natural or worked scarp that serves the same enclosing purpose. Modern field boundaries have cut through the monument at two points, the north-north-west and the south-south-east, disturbing the continuity of what would originally have been an almost perfectly circular boundary. A gap of just under three metres on the south-east side may represent the original entrance, the kind of narrow, controlled opening typical of ringfort design. Approximately forty metres to the south-east lies a graveyard, a proximity that is not especially rare in the Irish landscape, where early ecclesiastical and domestic or defensive sites frequently cluster, but which adds another layer of accumulated human use to this particular hilltop.