Simple in construction, it was built from flat slabs arranged to form a basin, and it was fitted with an overflow drain to prevent flooding. It is, in other words, a cistern, and the fact that whoever built it thought carefully about excess water as well as storage suggests a degree of practical engineering that sits quietly at odds with the romantic image of isolated island hermits living only by prayer and providence.

Church Island sits in Lough Currane on the Iveragh Peninsula, one of many sites across south Kerry where early Christian communities established themselves, often on islands or promontories that offered both separation from the wider world and access to fresh water, fish, and building stone. The cistern is associated with the broader monastic complex on the island, and its presence points to the logistical reality of sustaining a small religious community in such a location. Rainwater running off a wall or roof would have been channelled into the slab-lined pit, accumulating until needed, with the overflow drain ensuring the structure itself was not undermined by waterlogging. It is a modest feature, easily overlooked beside the more visible remains of church walls and enclosures, but it represents the kind of careful, unglamorous planning that made permanent habitation on a lake island possible at all.