Alexander Martin Sullivan, Serjeant-at-law, better known as A M Sullivan (14 January 1871 - 9 January 1959) was an Irish Lawyer, best known as the leading counsel for the defence in the 1916 treason trial of Roger Casement.
A younger son of A M Sullivan, he was born in Dublin and educated at Ushaw College, Belvedere College, Trinity College Dublin and King's Inns. Sullivan was called to the Irish Bar in 1892 and practised on the Munster Circuit. He was appointed an Irish KC in 1908 and King's Third Serjeant at Law in Ireland in 1912 advancing to Second Serjeant in 1913 and First Serjeant in 1921 - the last holder of that position.
A moderate constitutional nationalist and supporter of the Irish Parliamentary Party, Sullivan was a prominent campaigner for Irish recruitment 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 at least one attempt on his life. 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. He ceased practising law in 1949.
Sullivan was considered one of the most formidable advocates of his generation. In addition to his legal career, Sullivan authored of two books: Old Ireland in 1927 and The Last Serjeant in 1952.