It looks like a natural feature, soil-covered and overgrown, but it is a crannog, an artificial island built by human hands and inhabited at some point in Ireland's past. Crannogs were constructed across Ireland and Scotland from the Bronze Age onwards, often serving as defensible homesteads, their watery surrounds offering security that a landlocked settlement could not. This one, on the southern shore of the lough near Ballagh, measures approximately 17.8 metres east to west and 16.2 metres north to south, and it sits in somewhere between 1.1 and 2 metres of water depending on the season.

When surveyed by Crumlish in August 1996, the crannog rose just 0.75 metres above the lake surface, which is considerably less than its original height. Stones and flags have been removed over the years, mostly from the outer wall-face, gradually reducing what would once have been a more substantial structure. A narrow plinth, surviving between 0.1 and 0.2 metres wide, could still be traced running from the north-west around to the north-east. The interior, covered in soil and vegetation, contained a small drystone structure, and on the eastern side there was a patch of disturbance roughly 4 metres across and 0.6 metres deep, the apparent remains of an attempt to create a landing area for boats. About 60 metres to the south-south-west, a possible causeway juts out from the shoreline into the lough, likely the original approach route to the island. A causeway of this kind, linking shore to crannog, was a common arrangement, allowing access while keeping the island itself isolated enough to be defensible or at least private.