Or rather, it was recorded as existing, was searched for, and was not found. That particular category of absence, a structure noted in the historical literature but leaving no physical trace, gives Derreens Island, known in Irish as An Toileán Rua, an quietly unsettling quality.

The island sits at the southern tip of the larger landmass, and it is home to the remains of an old church. In 1991, a researcher named Noone recorded that approximately sixty feet to the south of that church there had once stood a small dwelling, most likely intended for a priest or a religious community attached to the church. The suggestion is plausible enough; such modest clerical residences, built close to early ecclesiastical sites, are known elsewhere in Ireland. But when surveyors examined the area, the island was so densely covered in ferns and scrub that the ground around the church was almost impossible to read properly. Along a roughly twenty-metre band of clearer, stony ground near the shoreline, some twenty-five metres south of the church, nothing was found. No footings, no collapsed walling, no outline in the vegetation. The structure does not appear on the Ordnance Survey six-inch maps of 1838 or 1921, which means it either predates reliable cartographic recording or was never substantial enough to be marked. At the northern end of the same island, by contrast, the ruins of vernacular stone-built houses survive clearly, their presence suggesting that Derreens Island did once support a small settled community, even if whatever stood near the church has since vanished entirely into the ferns.