The designation itself raises questions. In Irish archaeological usage, the word altar can cover a range of features, from prehistoric flat-topped stones with possible ritual associations to early Christian slabs used for open-air worship, or even natural rock formations that accumulated devotional significance over centuries. Without knowing which kind this is, the name alone carries a particular weight, suggesting that someone, at some point, considered this place set apart from ordinary ground.

Furgill sits in a part of Mayo where the landscape has been shaped by both deep geological time and long human presence. The west of Ireland retains a remarkable density of named and half-named places, sites that appear in townland records and on older maps but whose full histories were never formally written down, or were written down and then lost. An altar in such a setting might mark the site of a Mass rock, a flat stone used for clandestine Catholic worship during the Penal era when public religious practice was suppressed, or it might be considerably older. The record exists, the classification exists, but the detail that would allow a more confident account has not yet been made available in any accessible form.

What can be said with honesty is that this is one of a very large number of Irish monuments whose formal documentation remains incomplete. The site is recorded, it has a name that implies accumulated human meaning, and it sits in a county where the ground has been read and reread by generations of people who found significance in particular stones and slopes. That, for now, is the extent of what the record yields.