O'Callaghan, Michael (1879–1921), nationalist and politician, was born 6 September 1879 in Eden Terrace, Limerick, son of Edward Hackett O'Callaghan, owner of the city tannery at Lower Gerald Griffin St., and Ellen O'Callaghan (née Smithwick), a member of the Kilkenny brewing family. His grandfather Eugene O'Callaghan was mayor of Limerick in 1864. A delicate child who suffered from asthma, he received no formal school education but read widely, especially history and economics. Having studied a four-year course at the Leathersellers College in London, he qualified as an industrial chemist and returned to Limerick to run the family business, later becoming managing director.
A nationalist with an interest in Irish culture, he learned Irish from the Limerick nationalist George Clancy (qv), and joined the Gaelic League. He also supported Irish industry and was a member of the Irish Industrial Development Association. A member of the first Sinn Féin club formed in Limerick in 1905, he was elected to Limerick city council for the Irishtown ward in 1911, transferring to the Abbey and Castle ward in 1920. In 1914 he joined the Irish Volunteers; after the 1916 rising he served as a treasurer of the National Aid Association and Prisoners’ Dependants Fund, and campaigned for Éamon de Valera (qv) in the August 1917 Clare East by-election. Unanimously elected mayor of Limerick in January 1920 following Sinn Féin's victory in the urban local elections (winning twenty-six of forty seats on Limerick city council), he held the post for one year. In his capacity as mayor he attended the funeral of the murdered mayor of Cork, Tomás MacCurtain (qv), after which he received a death threat. He received more death threats in 1921 and for a time moved out of his home for safety. On the night of 7 March 1921 he was shot dead in his home in the presence of his wife. On the same night his successor as mayor, George Clancy, and a Volunteer, Joseph O'Donoghue, were also killed. His killers were probably members of the Black and Tans or Auxiliary Division of the RIC; in an article in the New Statesman in 1961 Richard Bennett claimed that O'Callaghan was shot by an Auxiliary named George Nathan (later chief of staff of XV International Brigade in Spain; died at Brunete in 1937), while in 1982 the Limerick Leader identified two Black and Tans – Sgts Leech and Horan – as having been involved in the deaths.
On 30 July 1914 O'Callaghan married Kate Murphy (qv), lecturer at Mary Immaculate College; they lived at St Margaret's, O'Callaghan's Strand, and had no children. His widow became a prominent anti-treaty TD.