The building mixes round-headed and pointed arches in a way that confused even careful nineteenth-century observers, and one of its chancel windows has been quietly split apart by a tree growing directly through it. Locally it has always been known as Teampull na Trionóid, the Church of the Holy Trinity, a name that survives in oral tradition long after the Latin records that once tracked its affairs have been largely forgotten.

The church belonged to the parish of Villa Trostany, a name derived from the Thurstan or Trostan family who held lands here, and its vicar answered to the Rectory of Croom. It appears in the Black Book of Limerick, a medieval ecclesiastical register, and by the Royal Visitation of 1615 it was recorded as 'Trustanny alias Dunaman, residens, ad rectoriam de Croom.' The antiquary the Earl of Dunraven described it in 1865 in careful detail, noting that the walls are built of rough boulder stone throughout, with dressed sandstone reserved only for the quoins, doorways, and window surrounds. The two opposing doorways in the nave walls are segmental-headed, meaning their arches describe a shallow curve rather than a full semicircle, and the stonework of their jambs slightly inclines inward. Just inside the south doorway, a cut-stone stoup, a small basin for holy water, projects from the wall face. A row of corbels, stone brackets projecting from the wall, at the west end of the nave supported an upper floor that was almost certainly a residence for the priest. Dunraven placed the church's construction in the late twelfth century, though he acknowledged that Irish buildings of this transitional Romanesque-to-Gothic style can run somewhat later than their English equivalents. T. J. Westropp, recording the site in the early twentieth century, leaned towards a late fifteenth-century date for at least part of the fabric. The place-name itself is documented as Dunnemeaunn or Rustainy in 1410 and as Ecc. Ville Trostanii in 1418.

The church stands roughly 370 metres north-east of Dunnaman Castle, which is itself worth noting as it contains a sheela-na-gig, a carved stone figure of a type found on many medieval Irish ecclesiastical and secular buildings. The ruins are in grassland and ivy has advanced considerably since Dunraven's visit, covering the chancel arch, the east window, and several other features that were legible in 1865. What remains visible includes the two aumbries, small square recesses set into the chancel walls that once served as wall cupboards for liturgical vessels, the fragmentary stoup, and the corbels of the upper floor. Aerial photography has revealed earthwork traces of an earlier ecclesiastical enclosure in the fields immediately to the north, east, and south, suggesting the standing remains are only one layer of a longer history on this ground.