It looks, at first glance, like a mausoleum, which is precisely what it has become. What makes it stranger is that the walls it encloses are almost certainly those of a medieval church, one that was converted to serve the dead in a rather more literal way than most ecclesiastical buildings ever manage.

The place takes its name from the Irish Cillín Tiarna, meaning the little church of St Tiernach, and it sits within the parish of Killeentierna in the diocese of Ardfert, barony of Trughanacmy. By 1841, when the Ordnance Survey was gathering field notes across Kerry, the original medieval structure had been so thoroughly absorbed by its surroundings that the surveyor recorded there were no ecclesiastical antiquities in the parish at all, noting only that the site of the old Killeen, a small burial ground associated with an early church, was occupied by a modern Protestant church. That Protestant church stands in the northern quadrant of the graveyard; the medieval ruins shown to its south on the 1841 six-inch Ordnance Survey map are a different matter entirely. A survey carried out in 2011 found that the central ruin, though heavily altered, retains walls roughly two metres high, built from coursed dressed rubble limestone with dressed limestone quoins at the corners. A barrel-vaulted roof has been added in relatively recent times and clad in rubble limestone, giving the surface an unusually rough texture quite unlike conventional roofing. A probable original slit aperture survives in the north wall, its chamfered limestone surrounds still intact. A later opening in the east wall is associated with the Twiss family tomb. Stranger still, what appears to be a saddle quern, a flat stone used for grinding grain, projects from the south elevation, reused at some point as building material. The interior orientation runs north to south rather than the conventional east to west alignment of Christian churches, raising quiet questions about the building's original function or an episode of significant alteration.

Scattered through the graveyard are further fragments that appear to belong to the same structure: tooled limestone blocks serving as grave markers and plinth stones, a carved limestone cross-shaped piece that may once have been a keystone in a ribbed vault, and chamfered blocks that could have formed window sills or surrounds. The church itself has been rearranged so thoroughly over the centuries that the number of bays it once contained can no longer be determined. What remains is a single bay, its past uses layered one on top of another, each generation having found a practical reason to keep the walls standing.