It measures 4.35 metres long, 1.75 metres wide, and 0.8 metres thick, large enough to have served as an altar, and that is precisely what local tradition holds it to be: a mass rock, a place where Catholic priests celebrated Mass in secret during the Penal era, when public Catholic worship was suppressed under English law and the clergy were liable to arrest or worse.

Mass rocks are found across Ireland, typically in remote or sheltered spots, hillsides, bogs, coastal margins, where a congregation could gather with some hope of going undetected. The one at Bank Harbour has the added particularity of sitting within a natural frame of rock outcrop to the west, north-west, and north-east, geography that would have offered a degree of concealment from anyone approaching by land. What makes this site unusual is the relatively recent effort to preserve it: in 1988, local people surrounded the slab with large boulders specifically to slow its erosion by the sea. That act of protection, recorded through a conversation with Connie Murphy, a national teacher from Castletownbear, gives the site a layered quality. The stone itself may date its active use to centuries past, but the community's decision to physically defend it is a gesture from living memory, a sign that the place still carries meaning well beyond the merely archaeological.