It is a slab roughly one and a half metres long, pierced through with a hole for a gnomon (the projecting rod whose shadow does the timekeeping), and carved with a semicircle divided by five radiating rays. It is a functional object, or was once, designed to regulate the hours of prayer at one of the most remarkable monastic sites in Ireland.

When the antiquarian R. A. S. Macalister recorded it in 1916 and 1917, the sundial was still in something close to its original setting, mounted on the south wall of the nave of St Caimin's Church on Inis Cealtra, the island known in English as Holy Island, in Lough Derg on the Co. Clare shore. Inis Cealtra was a significant early Christian foundation, and a south-facing wall was the correct position for such an instrument; the gnomon would have cast its shadow across the carved rays to mark the canonical hours. Macalister noted it carefully, assigned it a catalogue number, and photographed it for the record. At some point after that, the slab was removed from the island and entered the care of the OPW, which is responsible for many national monuments, eventually coming to rest in the depot at Athenry. The move was presumably made to protect it, though the effect is that an object built to read sunlight now sits in storage.