This is a cillin, the Irish term for an unconsecrated burial ground used for unbaptised infants, and sometimes for others considered ineligible for burial in hallowed ground. For centuries, these sites occupied a strange threshold in the landscape, neither fully sacred nor entirely forgotten, tended by local communities even as the Church refused to acknowledge them. The one at Bleannagloos sits in grassland with a stream running along its western and northern edges, a natural boundary that gives the roughly subrectangular enclosure, measuring around 23 metres across and 18 metres deep, a sense of being held apart from the surrounding fields.

When it was formally inspected in July 1984, the site was overgrown, its earthen field bank still intact but the interior in a disordered state. The graves were indicated not by inscribed markers but by set stones of limestone and sandstone, placed haphazardly rather than in rows, which is characteristic of cillini across Ireland where the burials were carried out informally, often at night and without clergy. A reference to the site appears in O'Flanagan's 1927 work, suggesting it was known and noted well before any modern survey. By 1931, when the third edition of the Ordnance Survey six-inch map was produced, a south-eastern entrance to the enclosure was already marked, though by the time of the 1984 inspection it had fallen out of use and was identifiable only by a holly tree, a species long associated in Irish tradition with boundaries and liminal places. A local group subsequently undertook works at the site. Approximately 60 metres to the north lies St Brendan's Well, a holy well dedication that points to an older layer of sacred geography in this part of north Galway.