What makes the building quietly arresting is that its orientation, running roughly west-south-west to east-north-east, was not chosen by liturgical preference alone but dictated by the shape of the ground beneath it. The landscape, in a sense, designed the church.

The oratory measures 7.2 metres in length and 4.75 metres in external width, modest even by the standards of early Irish ecclesiastical architecture. A trabeate doorway, meaning one with a flat, horizontal lintel rather than an arch, opens in the west gable, and a flat-headed window pierces the east end. Two projecting corbels, stone blocks that once supported a timber or thatch roof structure, survive at the east and west ends of the north side-wall. Inside, an altar and an aumbry remain at the east end; an aumbry is a small wall recess, typically used to store sacred vessels or relics. Roughly twenty metres to the north-east, a holy well associated with the same dedication adds a further layer to the site's devotional geography. Holy wells in Ireland were frequently paired with early chapels or oratories, functioning as places of ritual washing or pilgrimage distinct from, but inseparable from, the religious enclosure nearby. The chapel is attributed to Saint Caillin, and the site is noted in sources going back to Hardiman's work of 1846 and O'Flanagan's survey of 1927, suggesting it attracted scholarly attention long before more systematic archaeological recording.

Chapel Island forms part of the Slyne Head archipelago, a scatter of low-lying islands and rocks extending into the Atlantic south-west of Clifden. Access to the island is by boat, and the exposed nature of the archipelago means conditions on the water can change quickly. The oratory sits on the eastern, more sheltered side of the island, and the hollow that cradles it would have offered some protection from Atlantic weather to whoever built and used it, a practical consideration that has also, inadvertently, helped preserve it.