That year saw the foundation of the Gaelic League, with Hyde as moving spirit and first president. The league flourished and by 1905 had 550 branches, which organised lessons in Irish and encouraged Irish dances and games. He went to America in 1905 on a successful fund-raising tour and on his return was made a freeman of Dublin, Cork, and Kilkenny.

The Gaelic League took a dominant part in the successful agitation to make Irish an essential subject for entrance to the National University of Ireland, founded in 1908. By this time it had become a vital force in the movement for national revival. Hyde became the first professor of Modern Irish in UCD in 1909 and held the chair until his retirement in 1932. He was a Free State senator 1925-26. He had always insisted that the Gaelic League remain a non-political organisation, but its very success in reviving the national spirit helped to inspire the separatist movement, and it became clear that the language and the political struggle could not be kept apart.

Hyde resigned the presidency in 1915 and, avoiding political activity, confined himself to academic pursuits. Characteristically, he showed no animosity towards his successors. When the office of President of Ireland was created under the Constitution of 1937, Hyde was unanimously selected by all parties and held office until his term expired in 1945. He died in Dublin, 12 July 1949. His wife, whom he married in 1893, predeceased him in 1939. They had two daughters.

Hyde's verse translations from the Irish have appeared in many anthologies. He adopted the pen-name An Craoibhín Aoibhinn, 'the delightful little branch'. His folk-tales kindled the imagination of Yeats, Lady Gregory, and others prominent in the literary revival. The IAL awarded him the Gregory Medal in 1937. His other publications include Religious Songs of Connacht (1906), A Literary History of Ireland (1899), and some one-act plays. His play Casadh an tSúgáin, produced by the Irish Literary Theatre in October 1901, was the first play in Irish to appear on a professional stage.