The ruins are those of St Finan's, a nave-and-chancel church whose two parts were built at different periods and in noticeably different styles, a distinction remarked upon by Ordnance Survey observers in 1841, who noted that the chancel was constructed of large stones set in lime and fine sand mortar while the nave used smaller stones bound with lime and coarser gravel. That distinction is now harder to read, as most of the walls are heavily mantled in ivy and survive only in fragments. The east gable of the chancel still reaches roughly 4.5 metres, and the east window, narrow and pointed on the exterior but with a round-headed arch on the interior face, remains largely intact. By 1898 that small window had been quietly transformed into a shrine: a figure of the Virgin in delftware placed at the centre, surrounded on the sill by broken rosaries, old scapulars, coins, buttons, crucifixes, fragments of human bone, and assorted worn trinkets left by visitors whose reasons, as one observer noted at the time, were "only known to the donors, if even to them."

The place carries its age in layers. The church of 'Keynmara' appears in the Ecclesiastical Taxation of the Deanery of Hacudoe for 1302 to 1306, valued at 13 shillings and 4 pence, with tithes of 16 pence per annum, which places it firmly within the organised medieval parish system of the Diocese of Ardfert. The dedication is to St Finan, and the church is reputed to have been associated with the Augustinian abbey on the island of Innisfallen in Killarney's lakes. After the Reformation, the church's revenues passed through several hands: in 1605 the moiety of the rectory was demised to a Thomas Springe, and in 1613 King James I granted the lands, including the advowson and right of patronage of the church, to Jenkin Conway. By 1615 the Royal Visitation of the Diocese of Ardfert confirmed Conway still held both this church and the nearby church of Killalee, with a Henry Reade serving as curate. The name Kenmare itself, from the Irish Ceann Mara meaning head of the sea or great river, originally belonged to this older settlement on the southern shore; it was only transferred to the plantation-era town founded on the northern bank in the seventeenth century. The medieval glebe land associated with the church, visible on both the seventeenth-century Down Survey map and the 1841 Ordnance Survey six-inch sheet, lay immediately to the south and west of the River Sheen where it meets the sea. That same ground is now partly occupied by the helicopter pad and car park of the Sheen Falls hotel.

St Finan's holy well lies roughly 70 metres to the northeast of the church, and in 1898 it was described as sitting on the shore itself, the tide ebbing and flowing around it, accessible only at low water. The graveyard, entered from the southwest corner, slopes steeply northward toward the estuary. A small chapel was also said to exist within the burial ground, supposed by Samuel Lewis in 1837 to have been built by Sir William Petty when the English colony was established in the area, though its precise location has not been identified.