Known in Irish as Carraig an Aifrinn, "the rock of the Mass", the stone once marked a place where Catholic worship was conducted in secret during the Penal Laws, the series of statutes that, through the late seventeenth and eighteenth centuries, severely restricted Catholic religious practice in Ireland. Mass-rocks were the improvised altars of that period, often flat or prominent stones in remote or sheltered spots where a congregation could gather with some hope of avoiding detection. This one carries a crudely carved Latin cross on one face, its arms formed by a low raised band roughly five centimetres wide and two to three centimetres proud of the surface, spanning almost the full width of the boulder.

The stone sits at the northern end of Inis Meáin, the middle of the three main Aran Islands, near the head of the new pier. The cartographer and writer Tim Robinson recorded it in 1980, numbering it 21 on his detailed map of the island, and placed it along the foreshore. By June 1985, when surveyors went looking, it could not be found at all. It was only on a return visit in February 2009 that it turned up again, this time resting on a pallet on a grassy verge on the north side of the road running to the pierhead. The most likely explanation is that it was displaced during construction work on the pier itself, the kind of quiet, unintentional relocation that happens when heavy machinery and old stones occupy the same ground.

The boulder is a substantial piece of granite, roughly ninety centimetres in each dimension, and the cross carved into it is nearly as tall as the stone itself. Visitors heading to the pier on the northern side of the island should find it beside the road, though its position on a pallet rather than bedded into the ground gives it an oddly provisional air, as if it is still waiting to be put back somewhere.