A cashel is simply a ringfort built from stone rather than earthen banks, the term deriving from the Latin castellum, and this one belongs to a type of monument that was once extraordinarily common across Ireland, numbering in the tens of thousands, serving as the fortified farmsteads of relatively prosperous families between roughly the fifth and twelfth centuries.

Coollagagh lies in the west of Mayo, a county where stone was always the more practical building material, and cashels here tend to blend into the surrounding terrain with particular ease, their walls sometimes reduced to a scatter of loose rock or a low, moss-covered arc that could easily be mistaken for a field boundary. The word ringfort covers a broad category, but the underlying idea is consistent: a roughly circular enclosure defined by one or more banks or walls, protecting a household, its outbuildings, and its livestock. Where the ground allows, a souterrain, an underground stone-lined passage used for storage or refuge, is sometimes found within such enclosures, though no specific features of this particular site are documented in currently available sources.