He was appointed an Irish
KC in 1908 and King's Third
Serjeant-at-law (Ireland) in 1912 advancing to Second Serjeant in 1913 and First Serjeant in 1919 – the last holder of that position. A moderate constitutional
nationalist and supporter of the
Irish Parliamentary Party, Sullivan was a prominent campaigner for the recruitment of Irishmen into the British army during the
First World War. His opposition to
Sinn Féin republicanism and his prominent role in conducting prosecutions on behalf of the Crown during the
Irish War of Independence led to an attempt on his life in 1920. In April 1921 Derry House,
Rosscarbery in County Cork, which Sullivan had purchased two years previously was burned by the IRA. As a result, Sullivan relocated to England in 1921 and established a career at the
English Bar, having previously been called to the
Middle Temple in 1899. He subsequently became a
Bencher and Treasurer of
Middle Temple. By courtesy, he was always referred to as Serjeant Sullivan, even though that rank no longer existed in England. He remained a member of the Irish Bar, and returned to Dublin at least once in 1923 to appear in the celebrated case of
Croker v Croker, where the children of the former leader of
Tammany Hall,
"Boss" Croker attempted to overturn his
will, which left his entire estate to their stepmother. Sullivan appeared for Richard Croker junior. ==Casement trial==