A fortified medieval tower rises from the north-west corner of the nave, its upper section quietly rebuilt in the nineteenth century; beside it stands a Board of First Fruits hall from around 1810, grafted onto stonework dating to roughly 1550; and attached to that is a transept added around 1860. The whole ensemble, dedicated to St Mary, sits in the north-west corner of an active graveyard, a few dozen metres from a stone castle to the north-north-east and an Anglo-Norman motte and bailey, an earthen mound originally topped with a timber fortification, a little further to the east. Three different modes of conquest and settlement, all within a short walk of one another.

The church was already a ruin by the early seventeenth century, a fact recorded with some terseness in the terrier accompanying the 1659 Down Survey map of Castletown parish, which describes the site as a 'ruined church a small castle in repaire with diverse small cabbins,' and notes that Delvin 'was formerly a market town and the Earle of Westmeaths cheife seate.' The 'small castle in repaire' is thought to refer to the medieval tower attached to the church itself. By 1837, Samuel Lewis could describe the building as 'a plain building of ancient date, but in excellent repair,' which suggests the circa-1810 reconstruction had done its job convincingly. That reconstruction incorporated the surviving fabric of the medieval church, including a two-light window under a segmental hood on the west wall, probably sixteenth or early seventeenth century in date, and a small round-headed window in the tower, blocked up but still visible. Measured in 1980, the nave ran some 32 metres east to west; beyond the present east wall, the grass-covered footings of the medieval chancel, the eastern liturgical arm of the building, could still be traced, extending a further 11.7 metres.

The tower itself rewards close attention. On its north-east corner, set into the outer wall, is a small carved stone head, the kind of detail easily missed from a distance. The south-west corner of the nave also carries a subtle anomaly: the south wall steps slightly outward from the general building line at that point, suggesting it was added or modified at a different period from the rest of the structure. The tower door from the nave interior has been blocked, and the roofing was removed sometime around 1955 to 1960, leaving the stonework open to the sky in the way that tends, over decades, to blur the boundaries between deliberate ruin and simple abandonment.