What sits on it is older and stranger than any county line: the remains of an early medieval oratory, a tiny rectangular structure measuring roughly six metres by four, with its west gable still standing to a height of about three and a half metres. The masonry is rough, there are no traces of window openings, and the whole thing gives the impression of something built quickly and plainly for a purpose that needed no ornament.

The island was known in early Irish sources as Inis Uachtair, a name that appears in the Martyrology of Donegal in connection with St. Carthach, a figure of some ecclesiastical weight. Carthach was a descendant of the Kings of Cashel, and according to the scholar Pádraig Ó Riain, he served as bishop of Seirkieran in Co. Offaly, the monastery founded by St. Ciarán, whose own successor Carthach was. That a man of such lineage and standing should be associated with a small island in a midland lake says something about the early Irish church's appetite for remote, liminal places as sites of prayer and retreat. By the medieval period, Inis Uachtair appears to have attracted pilgrims, and the island was considered part of the kingdom of Tethbha, a territory that broadly corresponds to modern Co. Longford.

Beyond the oratory itself, the island holds a few further fragments. To the south of the oratory stands the northeast corner of a later detached building, possibly a residence for whoever kept the site, rising to about a metre and a quarter. Nearby, a separate wall section roughly three and a half metres long and nearly a metre thick hints at a more substantial structure that no longer survives in any identifiable form. Popular tradition held that there was also a cemetery on the island, a belief borne out by the old Ordnance Survey maps, which mark it as such; where exactly the burials lie is now impossible to determine, though the more level ground to the northwest of the church is the most likely area.