Conjoined ringforts of this kind are relatively uncommon in the Irish landscape, where the far more typical arrangement is a single enclosed settlement standing alone in a field. Here, two distinct enclosures sit together, suggesting that whoever occupied this place considered the pairing significant, whether for reasons of family organisation, territorial division, or the need to keep people and livestock in separate but adjacent spaces.

A ringfort, or rath, is an earthen enclosure, typically circular or subcircular in plan, built during the early medieval period to define and protect a farmstead. The northern of the two enclosures at Cahernashilleeny measures roughly 42 metres east to west and 37 metres north to south, and is defined today by a single degraded bank. The southern enclosure is the larger of the pair, at approximately 51 metres by 48 metres, and was originally more elaborately constructed, with two banks and an intervening fosse, which is a ditch cut between earthen banks to add an extra layer of enclosure. Traces of the inner bank and fosse remain visible on the south-western and north-western sides, and the outer bank still survives along the south-eastern through to the western arc. Both enclosures are subcircular in plan, that characteristic slightly irregular roundness that distinguishes field-built earthworks from more formal geometric structures. A later field bank cuts across the monument at its south-east and south-west corners, a reminder of how subsequent centuries of agricultural use have gradually sliced through and obscured what was already a poorly preserved site.