It is not a formal ecclesiastical structure; rather, it is a low construction of crudely laid stones, measuring roughly 1.75 metres east to west and 1.65 metres north to south, and rising to a maximum height of about 1.2 metres. What lifts it out of the ordinary is a detail recorded by Aldridge in 1969: at each corner of the altar stood a curiously shaped water-worn stone, the kind smoothed by river or tide over centuries and then, deliberately, brought here and placed.

Children's burial grounds of this type, often called cillíní in Irish, were used across Ireland for unbaptised infants and others considered ineligible for consecrated ground under Catholic practice. They occupy a particular and melancholy place in Irish social history, frequently sited at the margins, in old ringforts, on townland boundaries, or near pre-Christian monuments. The altar at Largan Beg sits within one such space and carries an additional detail: a cross-inscribed stone rests upon it, a carved marker that suggests some form of devotional or commemorative use, however informal. The combination of the water-worn corner stones and the inscribed cross points to a layering of practice, where the instinct to mark, to sanctify, and to mourn found expression in whatever materials the land and shore could provide.