a market-town and parliamentary borough of Ireland, partly in King's and partly in Queen's County, on the Barrow, which is here crossed by two bridges, 44 miles W.S.W. of Dublin. The principal public buildings are two Protestant churches (one of which is a fine edifice with a lofty spire), a large Roman Catholic chapel, and a Methodist meeting-house, two national and two free schools, a market-house, dispensary, and savings-bank. Soap and candles are manufactured here; and there are eight annual fairs. The town was founded by Lord Arlington, from whom it derives its name; but afterwards it was forfeited to the crown, and under William III. it was settled by many French and Flemish Protestants, who built the two churches of the town. In one of these the service was conducted in French till 1817. Portarlington returns a member to Parliament. Pop. (1851) 2728.