ARDGLASS ("Green Height"), County Down, Ireland, 32m. S. by E. of Belfast on a branch of the Belfast and County Down railway. Pop. (1926) 634. Soon after the Norman invasion it became a busy port, a fact attested by the remains of no fewer than five castles in close proximity. There is an ancient church crowning the eastern hill and a curious fortified warehouse (called the New Works), dating probably from the 14th century, when a trading company was established here under a grant from Henry IV. Ardglass was a royal burgh and sent a representative to the Irish parliament. The chief industry is the herring fishery. Ard glass is a resort of visitors.