The water sits quietly in its roughly rectangular basin in County Monaghan, and the surface reads as unbroken. But when the level drops by roughly half a metre to a metre below normal, something emerges off the south-western shore: a low circular platform of large stones, about twenty-five metres across and only half a metre high, trailing a narrow stone causeway back toward dry land. For much of the year, it is simply submerged.

What surfaces is a crannog, or at least the remains of one. Crannogs were artificial or partially artificial islands, typically built out into lakes or marshes during the early medieval period in Ireland and Scotland, and used as defended homesteads or high-status residences. They were usually constructed from layers of timber, peat, brushwood, and stone, and were accessed by boat or by a narrow causeway of the kind that survives here, just about a metre wide and composed of the same large stones as the cairn itself. No timber structures remain visible at Annagose, only the stone core. The site is unusual enough that it appeared as a clearly marked island on the 1907 edition of the Ordnance Survey six-inch map, suggesting the water level at that time was low enough to expose it fully. On current editions, it has effectively disappeared.