On the top of the Greenan mountain, not far from the border of Northern Ireland, lies Grianan of Aileach, one of the finest stone forts in Ireland. 

The massive stonewall is 13ft thick and encloses an area 77ft in diameter. In the walls are small chambers; a series of stairs at regular intervals inside the walls gave access to the wall-walk. The entrance is very long and lintel led.

Legend says the ancient gods built it; the ring fort was known as the Sun Palace and was held sacred. Traces of ancient earthworks, dating to the early Iron Age, surround the fort, enclosing an area of about 5 acres. The fort itself was probably built in the early centuries of the Christian era. From the 5th to the 12th century AD it served as the royal seat of the O Neill sept of Aileach; it was destroyed by Murtogh O Brien, king of Munster in 1101. To make the demolition complete, the king ordered each of his soldiers to take away a stone from the fort. Grianan of Aileach was reconstructed by Dr Bernard of Derry in 1870, but archaeologists are doubtful about the inner restoration.