It measures fourteen metres long, eighteen metres wide, and rises to about one and a half metres, with an opening of nearly four metres facing north towards the water. The bog has preserved it well enough that its shape is still legible, and a small scatter of burnt material on the far side of the stream may represent a continuation of the mound's eastern arm, carried across by time or flood.

This is a fulacht fia, a type of monument found in considerable numbers across Ireland, particularly in low-lying or waterlogged ground. The term is used to describe a mound of fire-cracked stone, typically arranged around a trough, where water was repeatedly heated by dropping in stones that had been made red-hot in an adjacent fire. The cracked and spent stones were cast aside after each use, and over generations they accumulated into the characteristic horseshoe shape, with the open end facing the water source. The precise purpose of these sites has been debated at length; cooking is the most widely accepted explanation, though some researchers have argued for brewing, hide-working, or bathing. The Caherbirrane example follows the classic form closely, abutting a field fence on its northern side, with a drainage channel cutting through the southern edge of the mound, suggesting that managing water flow was a practical concern for whoever used this spot.