The earthwork is subcircular in plan, measuring roughly 39.5 metres north to south and 36 metres east to west, and it is the kind of monument that rewards a careful eye rather than a casual glance. Most of the enclosing bank has been lost, but enough remains along the arc running from the south-west through north to east to give a sense of the original structure.

A rath is an earthen ringfort, one of the most common monument types in the Irish landscape, typically built during the early medieval period as an enclosed farmstead for a family of some status. What makes the Fiddaun example quietly interesting is the detail that survives despite its damaged condition. Stone-facing is still visible on the inner face of the bank at the northern side, suggesting a more carefully constructed boundary than a simple earthen mound. On the eastern side, traces of the northern edge of what may have been a stone-lined entrance gap can still be made out, hinting at a formal threshold into the enclosure. Immediately to the south of this probable entrance, however, a large pit has been dug into the monument at some point, removing all trace of the bank in that area and leaving a gap in the record of the site that cannot now be recovered.