O’Hegarty, along with Gearoid O’Sullivan and Fionan Lynch, subsequently became members of the Committee of the Branch and, almost from the day they joined, their services were requisitioned for teaching Irish classes.

In 1913 he became a member and then stage manager of a troupe of Gaelic players, called Na hAisteoirí , relating historical traditions of Irishness, cultural revivalism, and nationalism.

[5] On 25 November 1913 Diarmuid Ó Hegarty, together with Gearoid O’Sullivan and Fionan Lynch, attended the meeting at the Rotunda Rink for the founding of the Irish Volunteers and they joined on that first night.

A disheveled appearance revealed a somewhat nervous disposition: "rapid utterance, hair flopped on forehead...untidy look, careless in dress.".

[7] Having failed to identify the importance of their prisoner, in error the authorities released him early, on 18 May, to return to his family and job in the civil service.

O’Hegarty was a central member of the IVDF and, when in September 1916 the two organisations amalgamated, O'Hegarty's influence helped to ensure that the new body, the INA&VDF, was dominated by republicans.

[10] On his release, O’Hegarty was prominent in the re-organisation of the Irish Volunteers after the 1916 Easter Rising, subsequently becoming a member of the executive of the IRB's supreme council along with Michael Collins and Seán Ó Murthuile.

[12] At Brugha's behest, Sean Ó Muirthile and O'Hegarty organized a first Convention of the Volunteers, which was held at Fleming's Hotel in Gardiner Street in October 1916.

Progress was slow for the next few months, but volunteer reorganization gained speed after most of the Rising prisoners were released from Frongoch internment camp in Wales on 23 December 1916.

Having been dismissed from the civil service for refusing to take the Oath of Allegiance to the Crown in 1918, his administrative talents found ample outlet in the secretariat of the revolutionary Dáil.

[17] O’Hegarty was very close to Harry Boland, a constant companion, and Michael Collins and, in 1918, this IRB triumvirate exercised considerable control in the nomination of Sinn Féin candidates for the General Election of December 1918.

The selection process was resented by those who had aspirations to enter the Dáil but who failed to be nominated and Páidín O’Keeffe, who was the full-time secretary of Sinn Féin from 1917 and had a fund of knowledge about the leaders, maintained that the vote against the Treaty was partly an anti-Collins vote, arising from the antagonism Collins, Harry Boland and Diarmuid O’Hegarty aroused by their choice of candidates for the December 1918 election.

[18] Within days of being appointed Minister for Finance in 1919 Collins set about raising the funds necessary if Dail Eireann was to fulfil its stated ambition of providing an alternative government to the British one that was operating from Dublin Castle.

A short propaganda film was produced, shot outside St Enda's, the school established by Pádraig Pearse, and it featured Michael Collins and Diarmuid O’Hegarty signing bond certificates to twenty-nine prominent subscribers.

Whilst in Mountjoy jail he became a dominant figure amongst the IRA prisoners, ordering Noel Lemass to end his self-imposed hunger strike.

Following the execution of Ned Daly in the aftermath of the Easter Rising O'Hegarty was made Commandant of the 1st Battalion of the Dublin Brigade of the Irish Volunteers.

Collins could be provocative towards his colleagues and, as O'Hegarty took as Director of Organization, he described O’Hegarty as follows: "a long cow-lick fell over his right eye; he had untidy collar angled tie and a disheveled appearance...worked hard...muttered rapid speech; mind worked quickly, shrewdly and surely...in clear clever imagery, often biting...quick intellect, often disguised by a surface casualness.

"[24]But this attitude came to exemplify the true heroic freedom fighter "Lack of general regard for health and personal comfort had become close to affectation with us; it was a sign of manliness.

O’Hegarty resigned his military duties in April 1921, being replaced as Director of Organisation by Eóin O’Duffy, to concentrate on his work in the Dáil secretariat.

In October 1921 he was a member of the delegation[24] appointed by the Dáil that went to London to negotiate[25] the Anglo-Irish Treaty[26] with the UK, serving as joint secretary.

Frank Pakenham in "Peace by Ordeal" describes Diarmuid O'Hegarty as, the "civil servant of the revolution," the man perpetually behind the scenes, could conceal from few who met him the gifts that were to make possible the Irish constitutional achievements at the Imperial Conference of 1926 '.

Shortly before de Valera resigned as President of the Dáil in January 1922, to be replaced by Arthur Griffith, GHQ had reassured him that the IRA would support the Government; but in reality it was as divided as Sinn Féin.

The clear victory for the pro-Treaty parties in the General Election of June 1922, reflecting a country tired of war and searching for peace, enabled O'Hegarty's appointment to the new Dáil Secretariat of the Provisional Government in 1922 – the beginnings of an Irish only civil service.

D wing, where many of the internees were, faced the North Circular Road where crowds would gather nightly to wave flags, sing songs of encouragement and shout messages.

When the internees refused, the soldiers opened fire on the windows at 3 p.m. on 14 July 1922, and George Plunkett and a volunteer called Kane were wounded.

[40] The Mutiny came to a head when the key mutineers scheduled a meeting in the Devlin's Hotel in Parnell Street on 18 March 1924 to inform the rank and file of their plan and how it was to be carried out.

[34] In the event, due to time constraints, none of the Generals in charge of the Army, the Minister for Defence or any member of the Executive Council were contacted until after the raid.

In March 1923, he was appointed Secretary to the Executive Council of the Irish Free State, leaving the army on 1 May 1923 to resume a civil service career full-time.

In 1927 he went to New York and Washington DC to represent the government at congressional hearings on the fate of republican funds in the USA which had not been paid into Irish accounts.

O'Hegarty's long connections with the revolutionary period, and the old guard, sealed his career in 1932 when a new constitution devised by De Valera had him removed from office.