A bullaun is simply a large rock, usually glacial or naturally occurring, into which one or more cup-shaped depressions have been ground, most likely by human hands over a very long period. These hollows were once used to grind grain or medicinal herbs, and in many cases became associated with ritual, with the collected rainwater in the basin believed to carry curative or cursing properties depending on the tradition. They appear across Ireland in their hundreds, often near early church sites or sacred enclosures, and their presence in a landscape tends to suggest layers of use stretching back well before the Christian period.

The townland name An Tuar Glas, meaning roughly the green bleaching green or green field in Irish, points to a place shaped by agricultural life over centuries. Bullaun stones in Mayo are not uncommon, but many remain poorly documented, sitting in field corners or beside ruined churches with little to mark their significance to a passing eye. This particular example at An Tuar Glas is recorded as a monument, which places it within a body of archaeological sites considered worthy of protection, even if the finer details of its dimensions, condition, and precise context have not yet been made widely available.

Because so little specific detail has been documented publicly for this site, a visitor would do well to approach with patience and local knowledge. Bullaun stones can be easy to overlook, often moss-covered and half-sunk into the ground, the basin worn smooth and shallow with age. The best way to locate one in an unfamiliar townland is usually to ask locally, since farmers and landowners in rural Mayo often know exactly where such stones lie on their land, even if they have no particular name for them.