Known as Crock Island, it is a crannog, an artificial or partly artificial island built for settlement, typically during the early medieval period, constructed by laying large stones as a foundation and piling brushwood and earth on top. What makes its situation quietly odd is that the lake around it was deliberately drained. Crock Island now sits on reclaimed land some 130 metres from the present shoreline, stranded in poorly drained rushy ground, its original purpose as a lakeside dwelling made legible only by the landscape it was once part of.

The draining of Lough Gur took place shortly after the first edition Ordnance Survey six-inch map was produced in the late 1830s, a detail noted by Harkness in 1869. On that earlier map, Crock Island appears as a feature on a western spur of the lake, lying just 70 metres east of the shoreline, with Grange Hill to the north, Knockfennell to the north-east, and Ardaghlooda Hill to the south. By the time O'Kelly surveyed it in 1944, the mound measured roughly 27 metres in diameter and stood about 2 metres high, its base formed by that circular platform of large stones beneath layers of brushwood and compacted earth. A further description in 1978 by O'Kelly and O'Kelly confirmed these dimensions and noted the platform's continuing presence above the surrounding land. The site is National Monument No. 247, and it sits within a dense cluster of archaeology: a ringfort and possible settlement platforms lie 260 metres to the east, Grange church and graveyard are 250 metres to the west, and out across the remaining lake, Garret Island holds a castle and a second crannog.

Access to the immediate area around Crock Island is complicated by the nature of the ground, which aerial photography shows as waterlogged and rush-covered, with the mound itself obscured by trees and scrub. Lough Gur House stands only 150 metres to the north, and the broader Lough Gur landscape is well signposted from the surrounding roads. The mound itself is not a formal visitor site, but its outline is visible on aerial imagery. Those familiar with the lake's archaeology will find it worth orienting themselves to where the old shoreline once ran, to appreciate just how far the water once extended and how deliberately it was pulled back.