Diarmuid O’Hegarty (Ó hÉigeartuigh), civil servant and revolutionary, is born Jeremiah Stephen Hegarty on December 26, 1892 in Lowertown, Skibbereen, County Cork, the eldest of seven children of Jeremiah Hegarty (1856–1934) and his wife Eileen (née Barry), both teachers. He is educated at the Christian Brothers school, St. Patrick’s Place, Cork, joins the civil service in 1910 and is posted to the Department of Agriculture and Technical Instruction, acquiring invaluable administrative experience as private secretary to T. P. Gill, secretary of the department.

O’Hegarty is a member of the Keating branch of the Gaelic League and the closely associated Teeling circle of the Irish Republican Brotherhood (IRB). In 1913 he becomes secretary and stage manager of a troupe of Gaelic players, Na hAisteoirí, which includes several who later become prominent revolutionaries: Piaras Béaslaí, Gearóid O’Sullivan, Fionán Lynch, and Con Collins. As second lieutenant of F Company, 1st Battalion, Dublin Brigade, Irish Volunteers, during the Easter Rising, he is in charge of barricades in Church Street, Mary Lane, Mary’s Abbey, and Jameson Distillery, an area which sees fierce fighting. Imprisoned in Knutsford (May 1-18), he is released in error and returns to his post in the civil service. On his return he is a key figure in the reorganisation of the Volunteers and IRB, becoming a member of the executive of the IRB’s supreme council along with Michael Collins and Seán Ó Murthuile. He also becomes a central figure in Kathleen Clarke‘s prisoner support group, the Irish Volunteer Dependents Fund, and when it amalgamates with the more moderate Irish National Aid Association to form the INA&VDF in August 1916, he helps to ensure that it is dominated by republicans.

O’Hegarty is very close to Michael Collins and Harry Boland and in 1918 this IRB triumvirate exercises considerable control in the nomination of Sinn Féin candidates for the 1918 Irish general election. In the same year he is dismissed from the civil service for refusing to take the oath of allegiance, but his administrative talents find ample outlet in the secretariat of the revolutionary Dáil and later in the service of the Irish Free State to such an extent that he has been called ‘the civil servant of the revolution’ and ‘the Grey Eminence of the Free State Government.’ As clerk of the First Dáil and secretary to the Dáil cabinet (1919–21), he is largely responsible for its success, organising meetings of the clandestine parliament and coordinating the work of various departments from his offices on the corner of O’Connell Street and Abbey Street and later in Middle Abbey Street. He is determined that the Dáil will demonstrate its worth by ‘functioning as any progressive government would be expected to function.’ He records the minutes and handles all correspondence of the Dáil cabinet. As the conduit through which the Dáil’s ministers communicate, his role is central to the effective operation of government on the run. The influence this gives him within the revolutionary movement is bolstered by his senior role within the IRB and the positions of military significance which he occupies. He is a member of the Volunteer Executive (Jun 1916–Nov 1921), Irish Republican Army (IRA) Director of Communications (Jul 1918–Mar 1920), and Director of Organisation (Mar 1920–Apr 1921). When convicted of illegal assembly and jailed in Mountjoy Prison (Nov 1919–Feb 1920), he immediately wields power within the prison, ordering Noel Lemass off a hunger strike.

O’Hegarty resigns his military duties in April 1921 to concentrate on his work in the Dáil secretariat and serves as secretary to the Irish delegation during the Anglo–Irish Treaty negotiations in London (Oct–Dec 1921). He is a vital voice for the Treaty within the IRB and is appointed secretary to the cabinet of the Provisional Government in 1922, participating in the unsuccessful army unification talks of May 1922. During the Irish Civil War he is briefly seconded from his civil service post to serve as military governor of Mountjoy Prison (Jul-Aug 1922), where he threatens that prisoners who persist in leaning out of windows and talking to the public outside the prison will be shot. Peadar O’Donnell, who is a prisoner there at the time, remembers him as the focus of much ‘republican bitterness.’ A member of the army council during the Irish Civil War, he serves as Director of Organisation (Jul–Dec 1922) and Director of Intelligence (Dec 1922–May 1923), leaving the army with the rank of lieutenant general on May 1, 1923 to resume his civil service career.

O’Hegarty is secretary to the Executive Council of the Irish Free State (1923–32) and principal private secretary to its president, W. T. Cosgrave. Again he records the cabinet minutes and is the administrative pivot upon which government turns. He serves as secretary to numerous governmental delegations and is widely praised for his work in this role at the 1926 Imperial Conference and the 1930 Imperial Conference. In 1927 he goes to New York to represent the government at a hearing into the fate of republican funds in the United States. His career is the prime example of the influence of revolutionary veterans within the higher civil service in the early years of the state. After the change of government following the 1932 Irish general election, he is one of the very few senior civil servants who is effectively removed from his position. He is appointed to be a commissioner of public works, becoming chairman in 1949, a position he holds until his retirement in 1957. In 1939–40 he serves on the Economy Committee established by the government to advise on wartime spending, and in 1941 is a member of a tribunal of inquiry into public transport, which is principally concerned with the poor financial state of Great Southern Railways.

On April 27, 1922, with Michael Collins as his best man, O’Hegarty marries Claire Archer, daughter of Edward Archer, a post office telegraph inspector from Dublin, and Susan Archer (née Matthews). Her brother is William (Liam) Archer. They live at 9 Brendan Road, Donnybrook, Dublin.

O’Hegarty dies on March 14, 1958 in Dublin, leaving an estate of £5,441. His papers are in the University College Dublin (UCD) Archives.