From the 1916 Rising, the troubled Treaty negotiations and the Civil War, right through to his retirement after a longer period in power than any other 20th-century leader, Eamon de Valera has both defined and divided Ireland. He was directly responsible for the Irish Constitution, Fianna Fail (the largest Irish political party) and the Irish Press Group. He helped create a political church-state monolith with continuing implications for Northern Ireland, the social role of women, the Irish language and the whole concept of an Irish nation. Many of the challenges he confronted are still troubling the peace of Ireland and of Britain, and some of the problems are his legacy. This biography is by the author of ''Michael Collins''.