What the historic OSi cartographers recorded as St Ann's Chapel is in fact the ruins of a much older foundation, one dedicated not to any Ann but to a sixth-century British bishop named Sanctan, whose name the centuries slowly eroded into something more familiar. The transformation is almost audible: Kilmasantan, Cill escuip Sanctain, Kill St Anne. As the scholar Shearman noted in 1876, it was "a very facile adaptation of sound." The same drift produced the nearby placename Bohernabreena, itself a mangling of the same saint's name. The ruins sit on a west-facing slope in Glenasmole, the upland valley of the River Dodder, in the eastern quarter of a raised graveyard that overlooks the river's eastern bank.

Sanctan was, according to the seventeenth-century hagiographer Colgan, a Briton by birth, his father Samuel described as a king of Britain, his mother Drechura a daughter of the King of Ulster. He is thought to have been among the many British missionaries who came to Ireland in the early sixth century, and his feast day falls on the 9th of May. By 952, the Annals of the Four Masters record the death of an abbot of Cill Easpuig Sanctain, suggesting the community he founded had persisted for several centuries. By 1179, the church and its lands formed a sub-manor of Tallaght under the See of Dublin, and the Ecclesiastical Taxation of 1302 to 1307 valued Kylmesantan at 100 shillings per annum. In 1192 the church was granted to St Patrick's Cathedral, and by 1615 the Royal Visitation recorded it as annexed to the vicarage of Tallaght, served by a curate named Thomas Drakeshawe.

What survives today is modest. The medieval church, built of randomly coursed granite apparently quarried from nearby Corrig Mountain, measured roughly 18 metres east to west by 5 metres north to south. Only a short section of the south wall still stands to any height, reaching about 3 metres internally, while the footings of the west and north walls are traceable at ground level. The east wall has vanished entirely. Inside the graveyard gate, immediately to the north of the ruins, a medieval font survives in situ. O'Hanlon, writing in the late nineteenth century, also noted broken granite crosses on the entrance piers, though these are no longer described as present. St Sanctan's Well, rebranded on maps as St Ann's Well, lies 185 metres to the northeast of the church remains and is worth seeking out as part of the same visit.