It is a mass-rock, one of hundreds scattered across the Irish landscape, each one a quiet remnant of the Penal era, when the practice of Catholicism was outlawed under a series of statutes introduced in the late seventeenth and early eighteenth centuries. With churches suppressed and priests either exiled or subject to arrest, congregations gathered outdoors, often in remote or sheltered spots, and a suitable flat rock would be pressed into use as the centrepiece of an open-air Mass. The priest faced real legal danger; so, in many cases, did the congregation.

The Penal Laws, passed by the Irish Parliament under pressure from Westminster, varied in their severity and enforcement over time, but at their height they prohibited Catholic worship, excluded Catholics from public office, and placed severe restrictions on land ownership and education. By the mid-eighteenth century enforcement had become inconsistent, and by the early nineteenth century many of the laws had been repealed or fallen into disuse, culminating in Catholic Emancipation in 1829. The mass-rocks that survive from this period are not grand or formally constructed features; they are ordinary stones that acquired an extraordinary function through necessity. Their significance is social and devotional rather than architectural, which is part of why they can be easy to overlook in a landscape full of more conspicuous monuments. The example at Glassillaun sits in a part of Galway where the Atlantic coastline and the scattered settlements of Connemara give the country a particular quality of exposure and remoteness that makes the image of a clandestine outdoor congregation feel entirely plausible.