The ringfort at Farranbrien is not remarkable for its grandeur; the enclosing bank, once perhaps a commanding presence, has been heavily worn down in places, and farm machinery has left its mark on the interior. What gives the site its quiet intrigue is the possibility of a souterrain beneath it, an underground stone-lined passage or chamber of the kind early medieval farmers sometimes built beneath their homesteads for storage, refuge, or both.

The fort itself is a rath, the most common type of ringfort in Ireland, formed by a roughly circular earthen bank and a shallow external ditch, or fosse, rather than by stone walling. This one measures about 38 metres across, a fairly typical diameter, with its entrance facing east-northeast. Thousands of such enclosures were built across Ireland during the early medieval period, roughly the fifth to twelfth centuries, serving as defended farmsteads for families and their livestock. Most were never grand fortifications in any military sense; the bank and fosse were as much a marker of status and a barrier against cattle theft as they were serious defences. At Farranbrien, the bank still reaches up to three metres at its highest external extent, which gives some sense of the original effort involved in throwing it up, even if much of it has since slumped or been disturbed.