LETTERKENNY, a market-town and river port of Ireland, county of Donegal, on the Swilly, 18 miles S.W. of Londonderry. It consists of a single street and a square, and contains an Established church, a Roman Catholic, and several Presbyterian chapels; a national and other schools, a fever hospital, dispensary, and bridewell. Considerable trade is carried on, chiefly in agricultural produce, at the weekly market on Friday, and in exportation by the Swilly. Pop. (1851) 2180.