The rath at Carnaun measures forty-three metres in diameter and is defined by two concentric banks with a fosse, or ditch, running between them. A causewayed entrance gap, roughly three metres wide, opens at the south, the kind of deliberate, formal threshold that distinguishes a rath from a simple enclosure.

A rath is an early medieval farmstead enclosure, typically dating from roughly 500 to 1000 AD, built to mark status and provide a degree of protection for a family and their livestock. The Carnaun example is in fair condition overall, though it has not escaped interference. Sections of the inner bank and the interior have been quarried away at the south-south-east, south-west, and north, leaving visible gaps in what would once have been a continuous earthen circuit. The monument has also been planted with trees, which both obscures its shape from a distance and, over time, can disturb buried archaeology through root action. That combination of quarrying and planting is a common story for ringforts across Ireland, many of which were treated as convenient sources of fill or drainage material long before their significance was widely appreciated.