Known as Seeaghanmnatee, this semi-circular enclosure curves around the edge of a natural hollow in the rocky pasture, its two concentric lines of stone packed with rubble between them. Roughly 27 metres in diameter, it opens out towards the lake rather than closing upon itself, and Professor M. J. O'Kelly, who recorded the site in 1944, noted that it does not appear to have ever been a complete ring. He also observed several similar arcs elsewhere on the hill, which raises the obvious question of what, exactly, these open-ended structures were for. No firm answer has been settled upon.

The site sits only 25 metres east of what is now the lake edge, but that proximity was once even more immediate. Before the drainage of Lough Gur in the nineteenth century, Seeaghanmnatee would have stood directly at the waterline, and the old shore edge, running north-east to south-west, is still traceable through the site itself. The enclosure does not appear on the Ordnance Survey six-inch map of 1840, but by the 1897 twenty-five-inch edition it is recorded as a small scarped oval mound annotated with its name. Knockadoon Peninsula has long been recognised as one of the more densely occupied prehistoric landscapes in Ireland. Excavations led by Seán P. Ó Ríordáin in the mid-twentieth century, and more recently examined by Cleary in 2018, have documented extensive Neolithic and Bronze Age settlement across the peninsula. Three of Ó Ríordáin's excavated sites lie within 100 metres of Seeaghanmnatee, and prehistoric settlement has also been identified on nearby Garret Island, roughly 120 metres to the north-west.

The enclosure sits in rocky pasture on a working agricultural landscape, so access requires care and consideration of land boundaries. The site is visible as an earthwork on aerial orthoimages, which gives a useful sense of its shape before arriving on the ground, where it reads more subtly as a low, curving rise in the terrain. The broader area around Knockadoon rewards slow exploration; Circle K, another of the peninsula's many recorded monuments, lies approximately 140 metres to the east-north-east. Those familiar with Lough Gur's more visited sites, such as the large stone circle near the lake's southern shore, may find the quieter north-western slopes of the peninsula less frequented and, in their own way, more thought-provoking for it.