Clermont Cairn, court cairn in County Louth, Ireland

Clermont Cairn is a round stone cairn on the summit of Black Mountain (Clermont Carn), 21 m in diameter and over 4 m high. In its southwest quadrant lie the remains of a megalithic structure about 3.5 m long, open to the southwest and narrowing from about 1 m to 0.5 m at the rear, with three lintels in position and the rear part corbelled. It was built in the early Neolithic, c. 4000–3500 BC.

The cairn was built in the early Neolithic, c. 4000–3500 BC, and belongs to the Clyde-Carlingford group of court cairns. It is a protected National Monument, though quarrying and blasting in recent decades have disturbed the site. Near its centre lie the remains of a 3.3 m dry-stone round building, possibly built by Thomas Fortescue, 1st Baron Clermont, who is also credited with the nearby stone circle.

The Irish name Carnán Mhaighréid Náir means 'cairn of noble Margaret', while the English name also recalls Lord Clermont of Ravensdale. The summit lies on the Táin Way long-distance walking route, and walkers pass the cairn en route. A modern stone circle and stone row stand nearby at Ravensdale.

The cairn sits on the summit of Black Mountain, reached on foot along the Táin Way (Slí na Tána) long-distance walking route through the Cooley Peninsula. The top is exposed at 510 m and the ground around the stones is uneven, so wear walking boots and carry rain gear. Quarrying and blasting have disturbed the site, so keep to the path and do not climb on the chamber stones.

Clermont Carn is the only main television transmitter in Ireland that is vertically polarised, and it feeds no relay transmitters. Its signal carries Saorview well beyond the border, and is picked up in Belfast. A 120-metre cable-stayed mast replaced the original tower in 2002; the old structure now handles microwave links and a 2-metre amateur radio repeater installed in 1982.