Ogham is an early medieval script, typically carved as a series of notches and scores along the edge of a stone, and used mainly to record personal names in an archaic form of Irish. The stone at Killemlagh Church, known in Irish as Cill Imleach, sits close to the northwest angle of the gable wall with its edge exposed to view, measuring 1.25 metres in length. A sequence of ogham-like scores is visible along that edge, reading left to right as CLHQOC, and further marks are partly discernible on the underside of the stone, which the wall itself obscures. The word "possibly" carries real weight here: the inscription has not been definitively read, and the stone's incorporation into the fabric of a later building makes proper examination difficult.

The church stands in a graveyard directly above Keel Strand, in a valley the locals call The Glen, overlooking St Finan's Bay and the distant outline of the Skelligs to the west. The site is traditionally associated with St Finan, as is a holy well located a short distance away, and the combination of that dedication with the probable ogham stone points to the likelihood that the medieval church was built on the site of a much earlier ecclesiastical foundation, one that may date to the early Christian period when ogham was still in use. The stone itself was almost certainly not carved for use in a church wall; it was most probably reused from an older context, as was common practice when later builders needed ready material. A detail recorded in 1902 adds another layer to the site's history: people afflicted with scrofulous diseases, a term for tubercular swellings of the lymph nodes often associated in folk tradition with the supernatural or the spiritually impure, once made rounds here in search of a cure.