The Irish Volunteers were formed after a speech given by John Redmond at Woodenbridge, Co. Wicklow on the September 20, 1914, in which he encouraged the Irish Volunteer Force to enlist in the British Army during World War I. Part of the Force supported Redmond's idea and formed the National Volunteers, while the remaining 13,000 members who were opposed became the Irish Volunteers. Although only a small minority, the Irish Volunteers were paramount in the Easter Rising of 1916 and many of the rebels involved in that campaign played an important role in the Irish War of Independence.
The official stance of the Irish Volunteers was that action would only be taken if the British authorities at Dublin Castle attempted to disarm the them, arrest their leaders, or introduce conscription to Ireland. The more militant Irish Republican Brotherhood (IRB), however, was determined to use the Volunteers for offensive action while Britain was tied up in World War I. Against the will of their chief of staff Eoin Mc Neill, the IRB's Military Council successfully infiltrated this force, intending to use it in a wartime rising. Their plan was to circumvent Mc Neill's command ordering a ban on military procedure on Easter Sunday, 1916, instigate a rising, and hope to get Mc Neill on board once the rising was inevitable.
Padraig Pearse, a committed Irish Republican and who had been an IRB member throughout, issued orders for three days of parades and maneuvers, a disguised order for a general insurrection. MacNeill soon discovered the real intent behind the orders and attempted to stop all actions by the Volunteers. He succeeded in delaying the Rising for a day, and limiting it to about 1,300 active participants, virtually all within Dublin. The Irish Volunteers meanwhile had been successfully infiltrated by the Irish Republican Brotherhood, and Mc Neill's authority was greatly compromised.
The Easter Rising was a failure, and large numbers of the Irish Volunteers were arrested, even ones that did not participate in the Rising. In the meantime, public support for the Rising was growing consistently and some of its members began making active preparations for a renewal of military action. Closer links had also been fostered with the increasingly republican Sinn Féin party. The increasingly repressive measures taken by the British government after the Rising drove the remaining Volunteers underground. With the election and establishment of the First Dáil Éireann, individual units of the Volunteers began, on their own initiative, to target members of the Royal Irish Constabulary in their vicinity, thereby invoking in a slapdash manner the War of Independence.
The volunteer commanders never fully accepted the central authority either of their own General headquarters (established in March 1918) or the political control of the Dáil government, though most took an oath of allegiance to the latter in August 1919. By then, the organisation was increasingly known as the Irish Republican Army, having in effect merged with the remnants of the IRB. This new force, its successors and its competitors retained the name of the Irish Volunteers by means of its title in Irish - Óglaigh na hÉireann. Michael Collins was the de-facto Commander in Chief of the force, while Cathal Brugha was the official Minister for Defence in the Dáil cabinet. The vast majority of the Volunteers were largely loyal to Collins, but more localised rebels such as Tom Barry had their loyal support bases.