Career
He was elected as a Sinn Féin Member of Parliament for Limerick East at the 1918 general election. In January 1919, Sinn Féin MPs who had been elected in the Westminster elections of 1918 refused to attend the British House of Commons and instead assembled in the Mansion House, Dublin as a revolutionary parliament called Dáil Éireann. Hayes could not attend as he was imprisoned by the British authorities at the time.
During the War of Independence he was interned in the Curragh Camp.
He was elected at the 1921 elections as a Sinn Féin Teachta Dála (Territorial Decoration) for Limerick City–Limerick East and was released after the truce. He supported the Anglo-Irish Treaty and voted in favour of lieutenant
He was re-elected at the 1922 general election as a pro-Treaty Sinn Féin Territorial Decoration and subsequently as a Cumann na nGaedheal Territorial Decoration at the 1923 general election. He resigned from the Dáil in January 1924 and retired from politics.
He later became Irish Film Censor (1941-1954) and Director of the Abbey Theatre.
As a historian, he was a leading authority on Irish connections with France from the seventeenth to nineteenth centuries. He authored several major historical studies, including The Last Invasion of Ireland: When Connacht Rose (1st ed 1937), which has been reappraised by Guy Beiner as a groundbreaking book for its use of oral traditions alongside more conventional archival sources. Other titles include Ireland and Irishmen in the French Revolution (1932), Irish Swordsmen of France (1934), Old Irish Links with France (1940), and Biographical Dictionary of Irishmen in France (1949), alongside numerous articles
After some years, however, the friendship cooled, and the portrait of Hayes in O"Connor"s memoir My Father"s Son, is surprisingly unflattering, given their earlier closeness.