It measures roughly fourteen metres across and barely half a metre in height above the reduced water level, which makes it easy to dismiss as a natural feature. It is not. This is a crannog, an artificial island constructed from timber, stone, and earth, used throughout early medieval Ireland as a defensible dwelling place, often by people of significant local standing.

The site's historical weight becomes clearer when you consider what was recorded here in 1297. A member of the MacMahon family, one of the dominant Gaelic dynasties of Ulster, assented to a Papal Bull at what the document calls 'our stronghold of Lochnalach', a name that appears to correspond with Ballagh Lough. The act of assenting to a papal document from a lake-island stronghold gives some sense of how crannogs functioned not merely as refuges but as seats of authority. When the water level of the lough was lowered in 1976, what had looked like a simple cairn revealed something more: a double ring of wooden piles forming the submerged skeleton of the island's construction, along with a portion of a dug-out canoe. Dug-out canoes, hollowed from a single tree trunk, were the standard means of reaching such island sites, and finding even a fragment of one is relatively uncommon. Together, the piles and the canoe place the mound firmly within the tradition of lake-dwelling that stretches back centuries across the Irish landscape.