ARDGLASS(the green height), a town of Ireland in Down county, Ulster, at the head of a small bay about 8 or 9 miles S. of the entrance of Strangford Lough, and 7 miles S.S.E. of Downpatrick, was a place of great importance soon after the Norman invasion. It had a considerable trade was a. royal burgh, and sent a representative to the Irish parliament. It is still remarkable for the ruins of five Anglo-Norman castles. Its permanent inhabitants, who are chiefly engaged in the fisheries, numbered only 613 in 1871; but its population during the bathing season is considerably increased by visitors. Ships of 500 tons may enter the harbour at all tides, and its inner cove admits vessels of 100 tons. There is a lighthouse at the end of the pier, in lat. 54° 15' N., long. 5° 36' W.