Roscommon, an inland Irish county, in the province of Connaught, having Sligo and Leitrim on the N., on the E. and S. Leitrim, Longford, Westmeath, King's County, and Galway, and on the W. Galway and Mayo. It is 60 miles long by 40 broad, and contains 607,691 acres. The surface is undulating or flat except in the N., where the Curlew and Braughlieve Mountains rise to 1,200 feet. The lower parts are fertile, and for minerals, coal and iron are worked to some extent, and limestone is quarried, while potter's clay and pipeclay are found. Oats and potatoes are cultivated, and many cattle and sheep reared. The Shannon drains the county, and there are several lakes. The town of Roscommon, which is the capital, is a corporate market-town, 79 miles N.W. of Dublin, and has the ruins of a fine thirteenth-century castle and abbey.