The largest of them, occupying the westernmost position in the group, is an unusually complete example of early Irish ecclesiastical architecture, and it rewards close inspection in a way that a casual glance might not suggest. Set within a double enclosure, it carries details that range from the plainly archaic to the quietly sophisticated, sometimes within the same wall.

The church consists of a nave and chancel, both built from large rectangular blocks laid in gently undulating courses. Entry is through a trabeate doorway in the west wall, a term for a flat-lintelled opening rather than an arched one, a form associated with early Irish stone churches. The same wall carries projecting antae, which are extensions of the side walls beyond the gable, another marker of early ecclesiastical building practice in Ireland. The south wall has two narrow round-headed windows with splayed ingoings, meaning the openings widen inward to draw more light. Between the nave and chancel stands a Romanesque arch, around four metres high and just over three metres wide. The architectural historian Tadhg O'Keeffe has noted that the arch resists easy dating because its simplicity could point to either an early or a late construction, a difficulty compounded by the fact that the wall around it was substantially rebuilt at some point. He favours a later twelfth-century date. In the internal corner where the south wall meets the west gable, a holy water stoup is set into the stonework. It is carved with two animals whose necks are intertwined, a small and striking detail in an otherwise spare interior. Beneath it sits a rectangular stone font. The settlement was associated, according to early sources, with three saints of the name Colman, a dedication that reflects the complexity of early Irish hagiography, where the same name was carried by dozens of distinct figures.

The three churches at Oughtmama are national monuments in State Care, and the site is accessible within the enclosure. The second church in the group lies only nine and a half metres to the east of this one, close enough that the relationship between the buildings is immediately apparent on the ground.