It is a crannog, an artificially constructed lake dwelling of the kind built across Ireland and Scotland from the Bronze Age through to the early medieval period and occasionally beyond, formed here from a roughly oval stone cairn measuring approximately 23 metres by 27 metres. The structure slopes steeply into the water at its edges, and by the time it was inspected in 1988 its surface sat about 1.3 metres above the waterline, largely obscured by trees. Animal bone found scattered in the water around the perimeter hints at the domestic activity that once took place here, the accumulated debris of meals and butchery that the lough has quietly preserved.

The island appears on Ordnance Survey six-inch maps from both 1838 and 1922, recorded simply as an island with no indication of its artificial origins. Whatever its ancient function, the site found a more recent purpose as a base for wildfowl shooting, and two small stone buildings along with a landing slip were added to the island in the intervening years. These later structures sit incongruously on top of the prehistoric cairn, a reminder that useful platforms in the middle of a lough tend to get reused regardless of what they once were. Perhaps the most quietly remarkable detail about the Kilgellia crannog is that it is not alone: two further crannogs lie within 170 metres to the west, meaning that Ballymore Lough once supported at least three of these artificial islands in relatively close proximity, suggesting a concentration of settlement or activity on this stretch of water that has received little wider attention.