The one at Ballyglass in County Sligo mostly conforms to that pattern, but at its south-eastern edge something different happens. The outer bank breaks away from its usual curving course and extends outward to enclose a distinct rectangular area, roughly twelve metres by ten. That kind of annexe, departing so deliberately from the circular plan, is not a common feature, and its purpose remains the sort of question that ringforts tend to leave unanswered.

A rath, as this class of monument is also known, typically consisted of a circular area enclosed by one or more banks of earth and stone, sometimes topped by a timber palisade, and served as the homestead of a farming family during the early medieval period, roughly the fifth to twelfth centuries. The Ballyglass example sits on the eastern side of a north-north-west to south-south-east ridge, positioned to overlook a bend in the Doonbeakin River some fifteen metres downslope to the north-east. The enclosed platform measures twenty-two metres in diameter. Its surrounding bank is largely levelled now, standing only about half a metre above the interior surface and just under a metre on its outer face, with a width of around four and a half metres. Further out, between three and four metres beyond the foot of that bank, faint traces of a second, similarly flattened bank survive, hinting that the enclosure was once more substantially defended. The south-eastern stretch of the outer bank is the best-preserved section, and it is here that the anomalous rectangular extension projects outward, its function, whether animal pen, garden enclosure, or something else entirely, left to informed guesswork.