Early in life he allied himself with the Home Rule movement, while concerning himself with the land issue, and in 1898 formalised that commitment by joining with William O'Brien in the United Irish League.

After he took his seat in Westminster, he rose on 19 February 1901 in the House and began his address in the Irish language, only to be ruled out of order.

It was the first time that Irish was used in a speech in the House of Commons in London,[5][6] and this rapidly made him a celebrity for the Gaelic Revival.

He was bitterly opposed to Sinn Féin, later after the founding of the Irish Free State he attacked W. T. Cosgrave and his Cumann na nGaedheal government as well.

O'Donnell then co-founded the National League Party with William Redmond, its aim to unite the country peacefully after the Civil War.