The burial mound at Annagh was once a substantial stony cairn, roughly 35 metres across and 5.5 metres high, raised in prehistory to mark, protect, or commemorate the dead. In 1977, road-widening work removed the entire south-western half of it. The cut exposed something that would otherwise have remained sealed: a cist, the term for a small stone-lined burial box or chamber, typically formed from large upright slabs topped with a capstone. Here, the cist was composed of at least six slabs and enclosed a chamber approximately 1.5 metres wide and 1.8 metres high externally, oriented roughly north-east to south-west.

Only three of those slabs remain in place now, and even those have partially slipped out of position. Local accounts noted at the time that a quantity of bone was removed from the chamber during the road works, though the details of what was found, and where it went, have not been formally recorded. What had been an intact, if unexcavated, prehistoric burial was opened not by archaeologists working carefully through layers of soil and stone, but by machinery widening a rural byroad. The mound that remained on the north-eastern side still sits on gently undulating farmland just off that road, a surviving fragment of something that was, until relatively recently, whole.