A crannog, to use the term, is a man-made or partially man-made island, typically constructed from timber, peat, and stone, used in Ireland from the Bronze Age through to the early modern period as a defensible dwelling place. The island recorded here was modest, roughly oval and no more than about fifteen metres at its widest point. A second small island, possibly another crannog, lay around ninety-five metres to the north-north-west.

What makes this site quietly strange is how completely it has been erased, not by destruction but by drainage. The Ordnance Survey's six-inch map of 1838 clearly shows the island sitting in open water near the lake's western edge. By later map editions, it had already ceased to be shown as such; the margins of the lake were instead depicted as marshy ground. Since then, water levels in Derrykin Lough have continued to fall, and the former shoreline has receded by perhaps twenty metres from where it sat in 1838, leaving behind a margin of wet, reedy pasture. The island, no longer surrounded by water, is no longer visible as anything distinct at all. A fulacht fia, a type of prehistoric cooking site typically identified by a mound of fire-cracked stone near a water source, has been noted a short distance to the north-west, suggesting the broader area carries a deeper archaeological presence than its quiet, grassy appearance now implies.