A rath, as these earthwork enclosures are known, was typically a circular raised platform defined by one or more earthen banks, used as a farmstead or defended homestead during the early medieval period, roughly 500 to 1000 AD. Thousands survive across Ireland, but this one at Ballyglass has had an unusually practical afterlife: its outer bank has been cut back to a near-vertical face and clad with stone, converting an ancient boundary into a functional field wall. The rath was simply too useful to be left alone.

The enclosure measures roughly 24 to 26 metres across and is defined by an earthen bank that still stands up to 2.1 metres high on its south-west side, though it has been worn down to a lower scarp on the north-east arc. A shallow depression running along the outer edge between the south-east and south-west may represent what was originally a fosse, the term for the ditch that typically accompanied such a bank and added to the defensive or demarcating effect of the whole structure. The interior slopes gently downward toward the east, and on that eastern side a section of scarp with relatively modern stone cladding may conceal what was once the original entrance, positioned to face the easier gradient of the knoll. A few hawthorn trees ring the perimeter, and a disused railway line lies about 50 metres to the west, a reminder that the land here has seen more than one era of construction come and go. A second rath survives 265 metres to the south-east, suggesting this corner of Mayo supported at least two neighbouring early medieval farmsteads within sight of each other.