O'Kelly was elected Sinn Féin MP for College Green in the 1918 British general election. Along with other Sinn Féin MPs he refused to take his seat in the British House of Commons. Instead they set up an Irish parliament, called Dáil Éireann, in Dublin. Ó Ceallaigh wasCeann Comhairle(Chairman) of the First Dáil. He was the Irish Republic's envoy to the post-World War One peace treaty negotiations at Versailles, but the other countries refused to allow him to speak, because they did not recognise the Irish Republic