One of those boulders, probably the southernmost, is rectangular and nearly flat across its upper surface, and it is this one that is believed to have served as a mass-rock. The uncertainty surrounding which stone it actually is only adds to the character of the place, a site identified by the landowner and noted in a local history rather than excavated or formally confirmed.

Mass-rocks were outdoor altars used by Catholic communities during the Penal Law era, broadly the late seventeenth and eighteenth centuries, when the public practice of Catholicism was suppressed under a series of legislative restrictions imposed by the British administration. With churches either destroyed or forbidden, priests celebrated Mass in remote or concealed spots, often using a flat stone as the altar, with lookouts posted nearby in case of soldiers or informers. The rock at Derrynakilla sits roughly thirty metres west of a burial ground, which is itself a suggestive proximity; these places of worship and burial often clustered together in the landscape, sharing the same marginal, semi-hidden ground. The site is mentioned in a local history publication called Hidden Gold, edited by Julia Kemp and published in 1998.

The boulders themselves offer no inscription or formal marker, and the ambiguity around which stone served the purpose is part of what a visitor would encounter. The southern boulder, roughly rectangular and level enough to have functioned as a surface for liturgical objects, is the most likely candidate, but nothing has been confirmed archaeologically. The ash tree above them, whatever its current age, gives the group a sheltered, slightly enclosed quality that fits well with what such sites were chosen for.