The site, known as Lismore or An Lios Mór, is a circular bivallate rath, meaning a roughly circular enclosure defined by two concentric earthen banks and their accompanying ditches. Within it, two souterrains and a hut-site were once visible. Souterrains are underground stone-lined or earth-cut passages, typically associated with early medieval settlement and used for storage or refuge. At Knockavrogeen, both types were apparently present: one earth-cut, one stone-built. Neither can now be traced on the ground.

What makes the site quietly compelling is a detail recorded by the antiquarian John Windele in 1848. One of the souterrains, he noted, reputedly contained a stone bearing what he described as 'round letters'. The phrase almost certainly refers to an ogham inscription, ogham being an early medieval script in which letters are formed by a series of notches and strokes cut along a central line, typically found on standing stones across Munster and elsewhere in Ireland. Whether the stone survived, was removed, or was simply lost to time and overgrowth is not recorded. The two entrances Windele described, one positioned in the fosse (the ditch between the earthen banks) and one within the interior of the rath, suggest a deliberate and considered design, the kind of concealed access typical of souterrain construction. The rath itself sits high enough to command views in all directions, which tells its own story about why someone chose this particular hillside.