What survives at Knockereen is a circular cashel, a type of ringfort built from dry-stone walling rather than earthen banks, roughly 24.5 metres in diameter and set on a south-facing slope in open grassland. For much of its circuit, that walling has collapsed into a low spread of rubble; elsewhere, only a degraded scarp in the ground hints at where the enclosing wall once stood.

Cashels of this kind were built throughout Ireland during the early medieval period, roughly between the fifth and twelfth centuries, and typically served as enclosed farmsteads for a single family or small household. The dry-stone construction reflects the local availability of stone rather than any particular status, though some cashels were associated with people of middling rank within the Gaelic social order. At Knockereen, the structure is now poorly preserved, its original form readable mainly from the northern and southern arcs where the rubble spread is most coherent. About 120 metres to the east-south-east lies a separate enclosed site, suggesting that this area of north Galway once held a modest cluster of activity, though what relationship, if any, existed between the two enclosures is not recorded.