In Irish archaeological records, the term can refer to anything from a Mass rock, where Catholic worship was conducted in secret during the Penal Laws of the seventeenth and eighteenth centuries, to a prehistoric standing stone or slab that later generations interpreted through a religious lens. The name alone is a kind of puzzle, pointing to ritual significance without specifying which era claimed it or for what purpose.

Mass rocks are perhaps the most common category of site given this designation across Connacht. During the period when Catholic practice was suppressed under Penal Law, congregations gathered outdoors, often on remote hillsides or beside large flat stones that served as improvised altars. Over time, these places accumulated their own reverence, sometimes absorbing older associations with pre-Christian sites that had long been considered sacred ground. Marblehill, like much of this part of Galway, sits in a landscape where layers of settlement and belief have been folded into one another over millennia, and a name like "altar" tends to mark one of those folded points.

The source material for this particular site is thin, and it would be dishonest to elaborate further on its specific history, dimensions, or appearance without solid evidence to draw from. What can be said is that sites of this kind are often easy to overlook in the field, sometimes amounting to little more than a large flat stone in rough pasture, easily mistaken for field clearance or natural outcrop. The name, preserved in the record if not yet widely documented, is itself the most durable thing about it.