But in the townland of Behaghane on the Iveragh Peninsula, a boulder of precisely those dimensions served as an altar during the Penal era, when Catholic worship was driven outdoors and priests celebrated Mass in remote fields, on hillsides, or beside inconspicuous rocks. A recumbent slate slab lies beside it, bearing a number of lightly incised crosses, small devotional marks that suggest the site was used with some regularity and that those who came here wanted to leave something of themselves behind.

The mass-rock sits within a cluster of features that together speak to a much longer history of sanctity in this small corner of south Kerry. About 27 metres to the west lies Tobar an Bhile, a spring well covered by a low drystone enclosure, with a cross-incised slab placed on top. "Tobar an Bhile" translates roughly as the well of the sacred tree, and the name is apt: an ash tree formerly grew beside it, old enough to be marked simply as "Old Tree" on the first edition of the Ordnance Survey map. Local tradition held that this tree marked the grave of St Crohane, the early Christian saint to whom the nearby medieval church and graveyard at Kilcrohane are dedicated. Holy wells in Ireland were often associated with pre-Norman saints, and the combination of well, ancient tree, and church here suggests a place that accumulated religious significance across many centuries, long before the Penal Laws made open worship dangerous.

What makes the site particularly affecting is the way its different layers of use sit so close together. The medieval church, the saint's well, the memory of a venerated tree, and the clandestine altar occupy a space no larger than a modest field. The crosses incised on the slate beside the mass-rock are lightly done, easy to miss, which seems in keeping with the circumstances under which they were most likely made.