At the start of the 20th century, the IRB is a stagnating organisation, concerned more with Dublin municipal politics than the establishment of a republic. A younger generation of Ulster republicans aim to change this and, in 1905, Denis McCullough and Bulmer Hobson found the Dungannon Clubs, whose purpose is to discourage enlistment into the British Army and encourage enlistment into the IRB.
In 1909, Michael Collins is introduced to the brotherhood by Sam Maguire. By 1914, the Supreme Council is largely purged of its older, tired leadership, and is dominated by enthusiastic men such as Hobson, McCullough, Patrick McCartan, John MacBride, Seán Mac Diarmada, and Tom Clarke. The latter two are to become the primary instigators of the Easter Rising in 1916.
Following the Rising some republicans, notably Éamon de Valera and Cathal Brugha, leave the organization as they view it as no longer necessary since the Irish Volunteers now perform its function. The IRB, during the War of Independence (1919-1921), is under the control of Michael Collins, who initially is secretary and subsequently, on June 28, 1919, is made president of the Supreme Council.
When the Anglo-Irish Treaty is signed on December 6, 1921, it is debated by the Supreme Council, which votes to accept it by an 11-4 vote. Those who oppose the Treaty include former leader Harry Boland, Austin Stack, and Liam Lynch. Anti-Treaty republicans like Ernie O’Malley, who fought during the Civil War against the Treaty, see the IRB as being used to undermine the Irish Republic.
The IRB becomes quiescent during the Irish Civil War, which ends in May 1923, but it emerges again later that year as a faction within the National Army. It supports Minister for Defence Richard Mulcahy against the “Old IRA,” which fights against the recruitment of ex-British Army personnel and the demobilization of old IRA men. This comes to a head with the Army Mutiny of 1924, in the wake of which Mulcahy resigns and other IRB members of the army are dismissed by acting President of the Executive Council Kevin O’Higgins. The IRB subsequently dissolves itself, although it is not known whether a formal decision is made or it simply ceases to function.