The rath at Rathdonnell is something more considered than that. It sits on a prominent rise in undulating pasture in County Mayo, and it is encircled not by one earthen bank but by three, each separated from the next by a fosse, the broad ditches that gave the defences both physical depth and a degree of psychological weight. That triple-enclosure arrangement, known as a multivallate rath, is comparatively uncommon, and it generally signals a site of some local importance, whether that meant wealth, status, or a genuine need for elaborated protection.

The innermost enclosure is a broadly oval raised area, roughly 40 by 45 metres, defined by an earthen bank nearly six metres wide and standing almost three metres high on its outer face. The entrance faces east, where a gap just over four metres wide cuts through the inner bank, with intermittent stone facing still visible along its sides. A ramp-like causeway carries the approach across the innermost fosse, and the passageway continues through a corresponding gap in the second bank, then widens considerably to a broad seven-metre opening through the outermost bank, flanked by sod-covered stones. The outermost fosse and bank survive only as an arc running from the north-northwest to the east; a road skirting the western side has truncated and levelled what once continued around the circuit. The site also takes deliberate advantage of its setting: the slopes to the west and south are steep, and low-lying wet ground to the south-southeast would have made approach from that direction awkward. Inside the enclosure, the southwestern quadrant sits at a slightly higher level than the rest of the interior, and near its northern edge there is a shallow circular depression that may mark the entrance shaft of a souterrain, an underground passage or chamber of the kind commonly used in early medieval Ireland for storage or as a place of refuge.

From the rath, two further monuments are visible without moving far. Another multivallate ringfort lies around 200 metres to the south, and a standing stone stands roughly 500 metres to the southeast. Whatever the original relationship between these sites, the clustering suggests this particular patch of Mayo was once a well-organised and perhaps well-populated landscape, the details of which have long since retreated under pasture.