MacMahon, Seán Michael (1893–1955), revolutionary and army officer, was probably the John Michael McMahon born 15 September 1893 at 118 Cork Street, Dublin, son of Edward McMahon, labourer, and Mary A. McMahon (née Delany). On leaving school he worked for the postal service, and then as a clerk in a Dublin firm. A member of the IRB, he joined the Irish Volunteers on their inception (November 1913), and was made a lieutenant in 1914, serving with the 3rd Battalion of the Dublin City Brigade. Promoted to captain of the battalion's B Coy, during the Easter 1916 rising he fought in the Ringsend area, and took part in the defence of Boland's bakery under the battalion commandant, Éamon de Valera (qv). Early in the week, MacMahon's company were assigned to hold the rail line (part of which they sabotaged) between Boland's bakery and Westland Row station. During the fighting he carried a wounded man to St Vincent's hospital despite being under fire from British troops. After the Volunteers’ surrender he was interned in England, being held in Wakefield prison, Frongoch internment camp, and Wormwood Scrubs.
Released in the Christmas 1916 amnesty, he returned to Dublin, and became vice-commandant of the 3rd Battalion. He also worked on the editorial staff of the Nationality newspaper, which had been founded by Arthur Griffith (qv), who became a close friend. Also a close associate of Michael Collins (qv), in 1919 he was attached to the Volunteers’ general headquarters as quartermaster general. Throughout the war of independence he organised supplies of arms and ammunition for the Volunteers, perfecting a sophisticated supply system which made use of sympathisers in the railway companies and shipping lines. He organised a Volunteer company comprised of dockers and seamen, who conducted importation of arms from abroad, and ran the supply operation out of premises in St Stephen's Green and at 32 Bachelor's Walk, where he had supplies stores. Working under difficult circumstances, he was often hard pressed to find adequate supplies of guns and ammunition, and it was said that his hair turned white prematurely from the stress of his responsibilities. On one occasion, on reading a newspaper account that detailed how Volunteers had ‘riddled’ British soldiers with bullets, he burst out: ‘Riddled! Blast them! Where do they think we'll get them?’ (Coogan, 130). He took part in the Custom House raid (25 May 1921).
A supporter of the Anglo–Irish treaty, MacMahon remained QMG in the new national army. Viewing the drift toward civil war with extreme regret, he was a member of the army reconciliation committee that tried to effect a rapprochement between the differing factions (May 1922). By the time of the attack on the Four Courts (28 June) he was the national army's adjutant general. It is believed that after Collins's death in August 1922, MacMahon succeeded him as head of the IRB. In September 1922 he was appointed army chief of staff (1922–4), succeeding Richard Mulcahy (qv), who replaced Collins as commander-in-chief. Toward the end of the civil war, MacMahon led 2,000 troops in a sweep of south Kerry. On foot of a police report that claimed inter alia that the army was not doing enough to subdue the irregular IRA, in March 1923 he joined Mulcahy and the other members of the army council in submitting their resignations, which the Free State cabinet refused to accept, ordering the officers to remain in their posts.
A close friend and ally of Mulcahy, as one of the several IRB leaders who controlled the national army MacMahon became a bête noire of the clique of disgruntled officers who resented their exclusion from leading positions in the reorganised IRB and in the army, and opposed what they saw as the Free State government's dilatory policy on the national question. The simmering conflict came to a head early in 1924 when MacMahon oversaw a massive reorganisation of the national army, involving demobilisation of two thirds of the civil-war troop strength of some 50,000 officers and enlisted ranks. When dissident officers presented government with a mutinous ultimatum (6 March 1924), MacMahon advised that immediate action be taken against them, with ‘great tact and firmness’ (Valiulis (1992), 210), and supported Mulcahy's opposition to the government's policy of appeasing certain of the mutineers’ demands. Dispatched to maintain loyalty within the garrison in Cork, MacMahon was thus absent from headquarters when the army council, led by Mulcahy, ordered a raid on a meeting of the mutiny's ringleaders in Devlin's public house, Parnell St. (19 March), without consulting either the government, or the newly appointed general officer commanding the defence forces, Eoin O'Duffy (qv). When the government responded to the unauthorised action by demanding the resignations of the entire army council, MacMahon refused to comply, demanding to know the specific reasons for his dismissal, and the opportunity to defend himself. Thereupon, the government withdrew his commission.
MacMahon testified before the army inquiry committee, refuting allegations that the national army was being taken over by ex-British-army officers, and by Irish officers who had joined the IRA only after the July 1921 truce. The committee reported (June 1924) no evidence to support charges against MacMahon of muddling, mismanagement, and incompetence. Though the other dismissed officers were similarly exonerated, MacMahon was the only one to be reissued his commission. After serving as OC Southern Command with the rank of major-general, he resigned owing to illness in January 1927. He died 26 March 1955 of a coronary thrombosis at his home, Meadowbrook, Ballinteer Rd, Dundrum, Co. Dublin; the death certificate states that he suffered from encephalitis lethargica for thirty-two years. A widower, he was survived by two sons and one daughter. His funeral was attended by Taoiseach John A. Costello (qv) and other members of the inter-party government. In October 1956 a memorial Celtic cross was unveiled over his grave in Deansgrange cemetery by Mulcahy, then minister for education. The new bridge constructed in the 1980s over Grand Canal Dock and rebuilt in 2007 was named after him.