A low island near the mouth of Carlingford Lough is known as Blockhouse Island. Little is evident, today, of the building that gave it its name. The islet was previously known variously as Ennis Moore, Great Island and Nun’s Island.
The strategic importance of Carlingford Lough as a harbour, and the potential of the islets at its mouth as defensive points, is recognised as early as 1602 with the English commander Lord Mountjoy advising the Privy Council, ” … Unless the places mentioned are fortified and well garrisoned, we think there will be danger of invasion [from Spain]..[Carlingford needs] A fort to be cast up in the bay in the island there. This will secure the harbour against the enemy though… [if he] gets in past the island, he can capture Carlingford and Newry and so all the north east of the Bann.”
During the English Civil War, Colonel Mathew wrote in 1645 about this island to his commander, saying “I am hastening forward the fortifying of the Island… we have mounted the guns upon the fort…”
The fort was manned still in 1649, causing Cromwell to complain in a letter to the Speaker of the Dublin Parliament that “In the entrance into the Harbour Captain Fern, aboard your Man of War, had some danger, being much shot at from the Sea Fort… ”.
The ‘blockhouse’ would have been imposing being built of stone with an “apsidal-fronted tower (semi-circular face), originally three storeys high, measuring 11.1m by 5.2m internally with walls 0.9m thick and with a gun platform to the north” Source: Arch Survey of Co Down, 1966.
There was a door at ground level in the west wall with machicolation, which is a projecting structure that allows defensive fire to be directed downwards upon attackers. Another door at first floor level, presumably reached by a portable ladder, allowed access to the tower from the gun platform. The outer walls had loopholes at ground level for firing muskets out of and some of these covered the gun platform, lest an enemy landed there and attempted to capture the cannons.