What catches the eye, once you begin reading the stonework, is how much history has been folded into a relatively modest structure. The building you see is really two churches in one: an early nave was grafted onto an older chancel, the original west doorway was broken out to create a chancel arch, and the roof pitch was widened to accommodate the expanded width. That arch is now gone, replaced by a concrete lintel, and the whole ensemble carries the marks of centuries of repair, neglect, and ad hoc intervention.

The site traces its origins to a monastic foundation associated with the sixth-century saint Colman ua hEirc. The Annals of the Four Masters record the death of "Diarmuid of Cill Caisi" in 846 AD, identified by some as Colman's successor as abbot, though that reading has been disputed. The ecclesiastical ownership of the place shifted dramatically around 1190 to 1200, when a Norman landowner, Baldwin Niger, granted the church to the Fratres Cruciferi, a hospitallar order connected to the Priory of St John the Baptist outside Newgate in Dublin. The actual handover, though, did not happen until 1307 to 1308, when the Reverend Walter de Valle transferred it, with the Bishop of Lismore confirming the arrangement sometime between 1325 and 1354. Meanwhile, in 1260, the church appeared in a jurisdictional dispute between the bishops of Lismore and Cashel, and by 1389 it had been enlarged. After the Dissolution of the Monasteries in 1540, it was leased to a John White for thirteen shillings and fourpence. By 1615 the chancel had been repaired while the body of the church was described as decayed, and the Civil Survey of 1654 to 1656 still refers to "a little church roofed" at Kilcash.

The most rewarding detail for anyone who makes the visit is the Romanesque doorway in the south wall of the nave. Standing roughly 2.26 metres high and just over a metre wide, it retains three decorative orders carved in sandstone, including chevron ornament, though all are heavily worn. Antae, the projecting side walls characteristic of early Irish churches, may once have featured here too; a carved sandstone block in the north wall is thought to be a remnant of one. During excavation in the early 1980s, the original threshold of the doorway was uncovered, along with architectural fragments bearing Romanesque decoration retrieved from the north-east corner of the nave.