an island off NW Europe: separated from Great Britain by the North Channel, the Irish Sea, and St George's Channel; contains large areas of peat bog, with mountains that rise over 900 m (3000 ft) in the southwest and several large lakes. It was conquered by England in the 16th and early 17th centuries and ruled as a dependency until 1801, when it was united with Great Britain until its division in 1921 into the Irish Free State and Northern Ireland