The enclosure is barely readable as a structure from a distance, its defining wall reduced to a broad, sod-covered swell in the pasture grass, with stones poking through at odd angles. And yet the outline is unmistakably deliberate: a near-perfect circle, roughly 36 metres across, sitting on a level terrace along a ridge slope that opens onto long views to the south and south-east across the hilly terrain of County Mayo.

This is a cashel, the term used for a ringfort defined by a stone rather than an earthen bank. Ringforts were the dominant form of rural settlement in early medieval Ireland, typically enclosing a farmstead and offering a degree of social definition as much as physical defence. At Coolnaha, the wall originally stood with a dressed stone face, and along the north-east to east arc, a number of large horizontally laid stones may represent the surviving basal course of that facing. Elsewhere the structure has fared less well. On the upslope north-west side it has been worn down most severely, and along a stretch from south-south-east to west-south-west, the enclosure wall has been absorbed into a later field boundary, which follows the curve of the old structure without quite acknowledging what it is following. The interior slopes gently downward toward the south-east and shows no obvious entrance feature. More intriguing is the collapsed souterrain visible in the western half of the enclosure. Souterrains are underground stone-lined passages or chambers, commonly associated with ringforts, and thought to have served for storage, refuge, or both. This one has fallen in, leaving a visible depression rather than an accessible passage.