The piscina, a shallow stone basin used in Catholic and Anglican liturgical practice for rinsing sacred vessels, was relocated here in 1921 from the nearby ruins of Killalee church. That kind of quiet transplanting, an object lifted from one ruin and embedded into a living building, is easy to walk past without noticing, yet it makes the Fossa church an unusual kind of layered space.

The building itself has a history worth following. By 1837, when Samuel Lewis compiled his Topographical Dictionary of Ireland, it was already functioning as a chapel of ease for the parish of Aghadoe, meaning it served as a secondary place of worship for parishioners who lived at some distance from the parish church. The 1846 Ordnance Survey six-inch map recorded it as a T-shaped structure, which corresponds broadly to the cruciform plan visible today, a long east-west axis with transepts towards the eastern end. The western gable carries a limestone bellcote, a small turret-like structure built to hold a bell without requiring a full tower, and brick corner buttresses anchor the same end. Pointed windows run along the walls, and an entrance porch has been added to the south side. The overall effect is of a building that has been modified incrementally over a long period, each addition legible alongside the last.