impressive towers of granite known as the Back Castles Slieve Binnian is one of the Mourne Mountains in County Down, Northern Ireland, 9km north of Kilkeel. It is the third highest mountain in Northern Ireland at. The summit is broad and flat with rocky tors at the north and south ends, with the Back Castles, impressive towers of granite, in between. To the south-west is Wee Binnian (460m). It lies to the east of the Silent Valley Reservoir and the west of the Annalong Valley. The Mourne Wall also crosses over Slieve Binnian.It is situated in the townland of Brackenagh East Upper (379 acres), in the civil parish of Kilkeel and the historic barony of Mourne.Places of interestOverlooking Annalong Wood on the eastern slopes of Slieve Binnian is the disused quarry of Douglas Crag.What appears to be the remains of an abandoned quarrying village lies on the south eastern slopes of Slieve Binnan. Ruins of rock huts are spread across a landscape littered with part quarried rock.Binnian TunnelThe Binnian Tunnel (2.5 miles long) was built between 1947 and 1950/51 underneath Slieve Binnian, to transport water from the Annalong Valley to the Silent Valley. A workforce of 150 was involved in two tunnelling teams which started from opposite ends and met in the middle nearly 800m under the roof of the mountain. Having been carefully measuring their positions the whole time, they found they were only two inches off course when the two tunnels met.