A proponent of armed struggle against British rule in Ireland for most of his life, Clarke spent 15 years in English prisons prior to his role in the Easter Rising and was executed by firing squad after it was defeated.

[4] In 1883, Clarke was sent to London, under the alias of "Henry Wilson",[5] to take part in the Fenian dynamite campaign advocated by Jeremiah O'Donovan Rossa, one of the IRB leaders exiled in the United States.

At one meeting, John Redmond MP, leader of the Parnellite Irish National League, said of him: "Wilson is a man of whom no words of praise could be too high.

[7] Following his release in 1898, Clarke returned to Ireland where he was met by a number of welcome home parties in Dublin and Dungannon, and Limerick offered him the freedom of the city, which he accepted.

[8] Despite his well-wishers, Clarke found it difficult to gain employment in Ireland following his return and would make the decision in 1900 to emigrate to Brooklyn in the United States.

[8] In New York, Clarke worked for the Irish nationalist organisation Clan na Gael under its leader, John Devoy.

[8] After Clarke fell into ill health in late 1905, he gave up his job at the Gaelic American and moved with his wife to a 30-acre (120,000 m2) farm in Manorville, New York, and bought another 30 acres there in 1907.

In 1911 Clarke organised a gathering of Irish Republicans at the graveside of Wolfe Tone in Bodenstown Graveyard, County Kildare to counterprotest the royal visit of George V to Dublin.

It was also during this time that Clarke displayed a large banner in his shop reading "Damn your concessions, England, we want our country!".

In usual IRB fashion, Clarke joined both with infiltration in mind, hoping he would be able to take internal control and sway the election of officers.

Clarke felt that Sinn Féin, which at that time advocated for a form of home rule rather than outright Irish Independence, was "all right as far as it goes, but it doesn't go far enough".

The two of them, as secretary and treasurer, respectively, de facto ran the IRB, although it was still under the nominal head of other men: James Deakin and later McCullough.

[8] When the old Fenian Jeremiah O'Donovan Rossa died in 1915, Clarke used his funeral (and Pearse's graveside oration) to mobilise the Volunteers and heighten expectation of imminent action.

[14] In late 1915, Clarke used his connections with Clan na Gael back in New York City to arrange for German weapons to be shipped into Ireland to coincide with the planned uprising.

Clarke feared a repeat of the 1867 Fenian Rising which ended in disaster due to, amongst other reasons, poor planning and preparation.

[8] It was also in January 1916 when an agreement was reached between the IRB and the leading Marxist James Connolly and his Irish Citizen Army.

Although Clarke avoided publicity and attention by nature, his activities did not go unnoticed by Dublin Castle, the centre of British intelligence in Ireland.

Dublin Metropolitan Police detectives correctly realised that Clarke was the primary mover behind a planned republican revolution in Ireland and began to heavily monitor his activities.

[8] Clarke was located at headquarters in the General Post Office (GPO) during the events of Easter Week, where rebel forces were largely composed of Irish Citizen Army members under the command of Connolly.

It has been said that Clarke indeed would have been the declared President and Commander-in-chief, but he refused any military rank and such honours; these were given to Pearse, who was more well-known and respected on a national level following the O'Rossa Donovan speech.

Quinn states that despite his frail physical appearance and aversion to public speaking, Clarke can be described as the fiercest, most tenacious and most zealous of all the Easter Rising leaders.