[2][3] Baptised as John,[4] he was the eldest son of Samuel O'Kelly, a boot and shoemaker of Berkley Road,[5] by his marriage[6] to Catherine O'Dea,[7] and had three sisters and four brothers, two of whom were educated by Patrick Pearse[8] at St Enda's School.

O'Kelly joined the National Library of Ireland in 1898 as a junior assistant to T. W. Lyster, remaining there until 1902, and becoming a subscriber to the Celtic Literary Society.

He came to Griffith's notice the previous years joining the Irish Republican Brotherhood as a member of the esoteric Bartholomew Teeling Circle from 1901.

One acolyte campaigner was Thomas Kelly, who joined him in pressing the government for improved municipal drainage schemes for Dublin's slums.

In March 1915, O'Kelly went to New York City, to inform Clan Na Gael of the plans for a rising in Dublin by the IRB.

He was one of a handful of men who might have known of the "All-Ireland" Volunteer HQ at Athenry, County Galway, according to Liam Ó Briain involved in marshalling the rebellion in the western hills from Limerick across the Shannon.

[13] Thereafter Hobson's mysterious "disappearance" became the moment when "a devoted son" of Ireland was excluded from the movement; but O'Kelly may have saved his life.

[19] In his role as Secretary, O'Kelly was tasked with preparing the Sinn Féin Executive Council for the Dáil Éireann Constituent Assembly, which had been agreed at the party Ard Fheis in October 1918.

[23][24] O'Kelly was followed to Paris as envoy by the eminently better-qualified George Gavan Duffy, who was from a titled family of barristers and diplomats.

[25] In 1920, O'Kelly relocated to Italy, where he met with Pope Benedict XV, briefing the pontiff on the political situation in Ireland.

[26] At the same time, O'Kelly met with the future dictator of Italy, Benito Mussolini, who helped the Irishman and other Sinn Féin emissaries to source weapons for use by the IRA.

In 1926, when de Valera left Sinn Féin to establish Fianna Fáil, O'Kelly returned to Ireland and was appointed a vice-president of the new republican party.

In March 1927, he became editor of The Nation and played a significant role in building up support for the new party before the June 1927 general election.

O'Kelly earned a controversial reputation over his key role in attempts to publicly humiliate the then Governor-General of the Irish Free State, James McNeill.

Stunts such as withdrawing the Irish Army's band from playing at diplomatic functions which the Governor-General attended, or in one notorious case the sight of O'Kelly and Defense Minister Frank Aiken storming out of a diplomatic function at the French Legation when McNeill, the guest of honour, had arrived, damaged O'Kelly's reputation and image, particularly when the campaign backfired.

It is not known for certain, but suspicion rests on O'Kelly's membership of a Catholic fraternal organisation, the Knights of Columbanus, which de Valera suspected had a source in the cabinet.

Again, the justification for de Valera nominating one of his senior ministers for the presidency was rumours that someone in the cabinet was, either deliberately or accidentally, letting information slip to the Catholic Church through the Knights of Columbanus.

However, the apparent entry of the popular Lord Mayor of Dublin, Alfie Byrne, into the presidential race (in fact he eventually failed to get nominated) and the belief that neither O'Kelly nor any other politician could beat Byrne (ironically a close friend of O'Kelly) led to all party agreement, on the opposition Fine Gael's suggestion, that the office go to Douglas Hyde, a Protestant, as an appreciation for his contribution to Irish society.

O'Kelly's most famous faux pas occurred during a state visit to the Vatican City, when in a breach with the standard protocol, he told the media of Pope Pius XII's personal opinions on communism.

He convened a meeting of the Council of State in 1947, to consider whether Part III of the Health Bill, 1947 – which provided the basis for the Mother and Child Scheme – should be referred, but he decided against doing so.

On each occasion, the Taoiseach who advised him to do so (de Valera in the first and third cases, and John A. Costello in the other two) had not been formally defeated in a Dáil vote in a manner showing a loss of support by a majority of TDs.

However, Costello considered that the vote failed by accident (due to a mistake by the party whips), and opted to reintroduce the measure the following morning, rather than seek a dissolution.

He made a point of ensuring that his first state visit, following the declaration of the Republic of Ireland in 1949, was to the Vatican City to meet Pope Pius XII.

This visit created controversy when the famously talkative O'Kelly inadvertently revealed the Pope's private views on communism.

[40] The author, Monsignor Pádraig Ó Fiannachta, reported that President O'Kelly kept barrels of draught Guinness stout on tap in Áras an Uachtaráin.

In 1918, O'Kelly married Mary Kate, known as Kit, the daughter of John Ryan, a farmer of Tomcoole, near Taghmon, County Wexford.