O'Sullivan, Gearóid (1891–1948), soldier and politician, was born 28 January 1891 at Coolnagrane, Skibbereen, Co. Cork, fourth son among six sons and three daughters of Michael O'Sullivan, farmer, of Loughine, and Margaret Sullivan (née McCarthy) of Coolnagrane. It is uncertain when the ‘O’ prefix was included in his name. Christened Jeremiah but known in later life as Gearóid, he was an outstanding pupil at national school (where he became a class monitor) and secondary school in Skibbereen. Encouraged by his teachers, he acquired a love of the Irish language. Not yet ten, he joined the Gaelic League in Skibbereen in October 1900; he took part in the Oireachtas debates of 1909. In 1911 he qualified at St Patrick's College, Drumcondra, Dublin, as a national school teacher and taught at Kildorrery, Co. Cork, but returned to Dublin in 1912 to take up a post at St Peter's, Phibsborough. He took an honours degree in Celtic studies at UCD (1913), an H.Dip.Ed. (1914), and an M.Ed. (1915). At the same time, he was an organiser and teacher with the Gaelic League, a member of its Keating branch at Parnell Square, Dublin, and a founder of the League's fáinne proficiency badge.
He joined the Irish Volunteers (F Company, 1st Battalion, Dublin Brigade) at their foundation in November 1913, was ADC to Seán Mac Diarmada (qv) during the Easter rising of April 1916, and was ordered by Patrick Pearse (qv) to raise the flag of rebellion over the GPO stronghold in Dublin. Interned at Frongoch in Wales after the rising, he belonged to the IRB group of prisoners closely linked with Michael Collins (qv), a proximity that continued throughout the crisis years to follow. Released in the amnesty of December 1916, O'Sullivan intensified his Volunteer activity, playing a prominent role in Carlow Brigade, for which he was briefly detained while working as a teacher at Knockbeg College, Co. Carlow. When the Irish Volunteers became the IRA in 1919, he was arrested again and went on hunger strike at Mountjoy prison, which led to his release. Active throughout the war of independence (1919–21) and narrowly avoiding recapture during meetings with Collins, he joined the supreme council of the IRB in November 1921, remaining there for the remainder of his military career.
From February 1920 O'Sullivan replaced Collins as adjutant-general of the IRA, a position he retained until the Anglo–Irish treaty of December 1921 (which he supported), resuming it a month later as a lieutenant-general of the new National Army, responsible for personnel and promotions. He was also elected to Dáil Éireann for Carlow–Kilkenny in 1921 and again in 1922 (retiring in 1923). His intellectual and organisational abilities guaranteed that his position within the army was safe after the death (August 1922) of Collins, to whom he owed much for his initial rise to prominence. On 28 August he was appointed to the newly created army council, whose most draconian prerogative became the military execution of republican prisoners. After the civil war of 1922–3, wholesale demobilisation of officers and other ranks took place, but O'Sullivan and his council colleagues Richard Mulcahy (qv), Seán MacMahon (qv), and Seán Ó Murthuile (qv) survived the fiscal axe. Their privileged position angered some officers, led by Maj.-gen. Liam Tobin (qv), alarmed at the rate of demobilisation and the state's apparent abandonment of Collins's republican ideals. Through the Irish Republican Army Organisation, they deplored the devaluation of their pre-treaty IRA service and the retention of certain former British army officers and instructors. O'Sullivan's brief as adjutant-general had placed him in the role of personnel manager: as the Defence Forces (Temporary Provisions) Act, 1923, transformed the National Army into the defence forces of an Irish dominion, he was clearly in the sights of those who disagreed with how these forces took shape.
As demobilisation continued and former British personnel became more evident, O'Sullivan and his colleagues became targets of suspicion that a hostile IRB clique had controlled the army council since its formation after the death of Collins. Exaggerated or not, such claims precipitated the army crisis of March 1924, in which O'Sullivan personally ordered a raiding party under Col. (later Maj.-gen.) Hugo McNeill (qv) to arrest its leaders. To defuse the crisis, O'Sullivan and his army council colleagues were forced to stand down, while the arrested dissidents were summarily retired. The subsequent army inquiry (April–June 1924) absolved O'Sullivan and his colleagues of any wrongdoing, but their active military careers were over. O'Sullivan, however, was for some time secretary of the military service pensions board.
Civilian life treated him well, as he entered a legal career and in 1926 was called to the bar. In 1927 he was appointed judge advocate general and remained so until 1932. After the assassination of Kevin O'Higgins (qv), minister for justice, in July 1927, he filled the vacated Co. Dublin seat in a by-election in August, retaining it at subsequent elections until 1937. In August 1928 he was a Free State delegate to the Empire Parliamentary Association conference in Canada. Openly supporting Gen. Eoin O'Duffy (qv) and the short-lived ‘Blueshirt’ vanguard of the fledgling Fine Gael party during 1933–4, he pointedly refused to surrender his legally held revolver when gardaí demanded it as a precaution against a feared Blueshirt coup d'étât. In 1937 O'Sullivan became a barrister on the western circuit, and in 1940 commissioner for special purposes of the income tax acts, a post he held for life. He lived at St Kevin's Park, Dartry, Dublin, where he died 26 March 1948, aged 57. His military funeral to Glasnevin cemetery, with his coffin draped in the same flag that had covered the coffin of Michael Collins, reflected his high national profile.
He married first (1922) Maude Kiernan, sister of Kitty Kiernan (qv) and daughter of Peter and Bridget Kiernan (proprietors of the Greville Arms hotel, Granard, Co. Longford), whose family was closely involved with the Irish political leadership, notably Michael Collins and Harry Boland (qv). After Maude's death he married Mary Brennan of Belfast; they had three daughters and a son, all of whom survived him. O'Sullivan features prominently in Leo Whelan's (qv) group portrait (owned by the Mulcahy family) of the IRA GHQ staff at the time of the Anglo–Irish truce (July 1921), and he is commemorated in Co. Cork by a plaque at Skibbereen town hall.