A bullaun stone is a boulder with one or more hollowed depressions, found throughout Ireland and often associated with early Christian sites; the one here, attributed to St Bearchán, is now broken, but the shallow depression it retains was held, as late as the nineteenth century, to bear the imprint of the saint's head and to offer a cure for headaches.

Bearchán, known locally as Broghan or Brachan, was a figure of considerable medieval reputation. He founded a monastery at Cluain Sosda, the early Irish name for Clonsast, reputedly in the late seventh century, and was credited with prophetic gifts that earned him the epithets Bearchán the Prophet and Bearchán na Buile, meaning roughly "of the vision". In 1189, Gerald of Wales named him in his Expugnatio Hibernica as one of the four prophets of Ireland, which gives some measure of how seriously his reputation was taken beyond these particular fields. By the sixteenth century, the library of the Earl of Kildare contained a manuscript called Leabhar Bearcháin na Cluana, believed to have originated at Clonsast and ascribed to Bearchán himself. His feast day falls on the third or fourth of December. The church built in his memory was already reduced to a divided nave and chancel by the time Comerford measured it in 1883, recording external walls four feet thick and no surviving architectural features or headstones. What remains today is less still: scattered foundation courses of greywacke and limestone laid in the large, irregular blocks characteristic of cyclopean construction, a technique associated with early medieval building in Ireland. The maximum surviving length runs to roughly fifteen metres.

The site sits in open pasture, which means the wider landscape is legible in a way that enclosed or wooded sites are not. A cairn lies about a hundred metres to the south-west, and the thorn tree and the broken bullaun stone associated with the saint are around a hundred and ten metres to the south-east, making this a small but coherent cluster of early medieval survivals in a setting that otherwise gives little away.