That single intrusion captures something of what Cahernabrock is: a cashel, a type of stone ringfort defined by a drystone enclosing wall rather than an earthen bank, that has been quietly absorbed into the working agricultural landscape of County Mayo, its ancient fabric partially dismantled, partially buried under grass and centuries of field clearance, yet still legible to a careful eye.

The cashel is oval in plan, roughly 31.5 metres on its longer axis, with a wall originally constructed in the typical manner: a rubble core sandwiched between dressed inner and outer stone faces. The basal course of the external face was laid with noticeably large stones, a detail that suggests some deliberate effort at a solid foundation. By 1979, surveyors recorded a depression outside the eastern and southern wall that may represent the remains of a fosse, a rock-cut or dug outer ditch that would have added a further layer of enclosure. When the site was revisited in 1999, the southern portion of that depression had been infilled. Three stones set on edge at the east-north-east appear to mark the jamb stones of the original entrance, though the gap itself has long since closed. Inside the northern half, a D-shaped sod-covered mound around eight metres across may once have been a substantial internal structure, possibly extending into the southern half before the later property wall cut across it. Three shallow surface depressions in the north-west quadrant are thought to indicate a collapsed souterrain, an underground stone-lined passage of the kind frequently associated with early medieval settlement sites, used variously for storage or as a place of refuge. Elsewhere in the interior, a discontinuous line of stones protrudes from the turf with no clear explanation. The site commands wide views to the west and north across gently undulating limestone terrain, a characteristic that almost certainly influenced its original siting.