Among the low wall footings in a graveyard outside Oldkilcullen, Co. Kildare, there sits a granite slab nearly two metres long with a circular notch cut at one end and a circular perforation at the other. Its precise function is uncertain, though it has been identified as a possible threshold stone, the kind of object that accumulates quiet significance precisely because nobody can say for sure what it meant. The ruins around it are equally reticent, reduced mostly to the limestone footings of a nave and chancel, but they carry a longer institutional history than their present state suggests.

The site belongs to an early monastic foundation dedicated to St MacTail, and the dedication held throughout the medieval period without interruption, suggesting a direct line of continuity from the pre-Norman monastery to the later parish church. In 1179, Pope Alexander III confirmed the church to the monastic community at Glendalough, and it was later transferred to the Augustinian priory of Holy Trinity in Dublin, which retained it through the rest of the Middle Ages. By 1584 it was substantial enough to have five annexed chapels serving the surrounding area. An episcopal visitation of 1615 found the chancel already in ruins while the nave remained sound; by 1630, both were recorded as being in reasonable repair, and Francis Grose's 1791 illustration shows the building still roofed and carrying an additional structure on the north side. The Romanesque character of the original building is attested by a base plinth, decorated with an angle roll, which was first uncovered during excavations in 1939 and recorded again in situ by Seán Sourke in July 2019. The excavator in 1939 noted that the chancel arch base was of the bulbous type and closely resembled the equivalent feature at the Nuns' Church at Clonmacnoise, a twelfth-century Romanesque church on the Shannon regarded as one of the finer examples of the style in Ireland. Alongside the arch base, the 1939 excavations recovered worked fragments in two materials unusual in combination: a friable granite and a calciferous tufa, the latter a porous limestone sometimes favoured in medieval construction for its light weight.

The remains are a National Monument in state ownership and lie within an active graveyard to the north-east of the old village centre. The walls themselves are mostly levelled, but the western doorway survives as two splays, and the south wall of the nave retains its buttresses. The possible threshold stone, noted by O Carragáin in 2000, is worth looking for among the stonework; it is not obviously dramatic, but it is the kind of object that rewards a slow look.