It is a crannog, an artificial island of the kind built and inhabited in Ireland from the Bronze Age well into the early medieval period, constructed from layers of timber, peat, stone, and brushwood heaped up in shallow water to create a defensible, elevated living space. At Knockroe, the lake has long since dried away, leaving the mound stranded in damp pastureland, its origins barely legible to anyone walking past.

The site was first documented in 1964, when archaeologist Ó Riordáin recorded a raised, grassed-over circular mound measuring 18.5 metres in diameter and roughly 1.5 metres high, with stones breaking the surface at several points around its edge. Along its southern extent, three spaced wooden posts, each only about ten centimetres across and protruding just 20 to 30 centimetres above ground, were likely the remnants of a row of piles, the kind of structural timbers that once anchored the island's platform. Two objects found on or near the mound speak quietly to everyday life on the site. A fragment of a rotary quern, a hand-powered grinding stone used to mill grain, suggests domestic activity. More striking was a wooden vessel containing bog butter, a substance, usually churned dairy fat, that was buried in Irish bogs for centuries, possibly as a means of preservation or as an offering. This particular vessel was allowed to dry out after its discovery and subsequently fell apart, an unhappy end for what might have been a significant find. Ó Riordáin also noted a second possible crannog roughly 70 metres to the east, suggesting the area may once have supported more than one island settlement on the same body of water.

By May 1999 the mound could not be located during a ground inspection, which is a reminder of how thoroughly the landscape can absorb even a feature nearly two metres tall. Its outline does remain visible on aerial imagery, a faint circular impression in the fields at Knockroe, more legible from above than from any ground-level approach.