The revolutionary period in Irish history was the period in the 1910s and early 1920s when Irish nationalist opinion shifted from the Home Rule-supporting Irish Parliamentary Party to the republican Sinn Féin movement. There were several waves of civil unrest linked to Ulster loyalism, trade unionism, and physical force republicanism, leading to the Irish War of Independence, the Partition of Ireland, the creation of the Irish Free State, and the Irish Civil War.

Some modern historians define the revolutionary period as the period from the introduction of the Third Home Rule Bill to the end of the Civil War (1912/1913 to 1923), or sometimes more narrowly as the period from the Easter Rising to the end of the War of Independence or the Civil War (1916 to 1921/1923).

The early years of the Free State, when it was governed by the pro-Treaty party Cumann na nGaedheal, have been described by at least one historian as a counter-revolution.

Home Rule seemed certain in 1910 when the Irish Parliamentary Party (IPP) under John Redmond held the balance of power in the British House of Commons. The third Home Rule Bill was introduced in 1912. Unionist resistance was immediate, with the formation of the Ulster Volunteers (UVF). In turn, the Irish Volunteers were established in 1913 to oppose them and prevent the UVF introduction of self-government in Ulster. The Dublin lock-out in the same year led to creation of the Irish Citizen Army.

In September 1914, two months after the First World War broke out, the UK Parliament passed the Government of Ireland Act 1914, known as the Home Rule Act, to establish self-government for Ireland, but the act was suspended for the duration of the war. Irish nationalist leaders and the IPP under Redmond supported Ireland's participation in the British war effort, in the belief that it would ensure implementation of Home Rule after the war. A core of leaders within the Irish Volunteers were against this decision, but the majority of the men left to form the National Volunteers, some of whom enlisted in Irish regiments of the New British Army, the 10th and 16th (Irish) Divisions, the counterparts of the unionist 36th (Ulster) Division. Before the war ended, Britain made two concerted efforts to implement Home Rule, one in May 1916 and again with the Irish Convention during 1917–1918, but nationalists and unionists were unable to agree to terms for the temporary or permanent exclusion of Ulster from its provisions.^{[citation needed]}

The period 1916–1921 was marked by political violence and upheaval, ending in the partition of Ireland and independence for 26 of its 32 counties. A failed militant attempt by the Irish Volunteers and the Irish Citizen Army was made to gain independence for Ireland with the 1916 Easter Rising, an insurrection in Dublin. Although support for the insurgents was small, the execution of fifteen people by firing squad, the imprisonment or internment of hundreds more, and the imposition of martial law caused a profound shift in public opinion towards the republican cause in Ireland. In addition, the unprecedented threat of Irishmen being conscripted to the British Army in 1918 (for service on the Western Front as a result of the German spring offensive) accelerated this change (see Conscription Crisis of 1918). In the December 1918 elections, Sinn Féin, the party of the rebels, won three-quarters of all seats in Ireland. Twenty-seven of these MPs assembled in Dublin on 21 January 1919 to form a 32-county Irish Republic parliament. The First Dáil Éireann unilaterally declared sovereignty over the island of Ireland.

Unwilling to negotiate any understanding with Britain short of complete independence, the Irish Republican Army, the army of the newly declared Irish Republic, waged a guerilla war (the Irish War of Independence) from 1919 to 1921. In the course of the fighting and amid much acrimony, the Fourth Government of Ireland Act 1920 implemented Home Rule while separating the island into what the British government's Act termed "Northern Ireland" and "Southern Ireland". In July 1921 the Irish and British governments agreed to a truce that halted the war. In December 1921 representatives of both governments signed the Anglo-Irish Treaty. The Irish delegation was led by Arthur Griffith and Michael Collins. This created the Irish Free State, a self-governing Dominion of the Commonwealth of Nations in the manner of Canada and Australia. Under the Treaty, Northern Ireland could opt out of the Free State and stay within the United Kingdom: it promptly did so. In 1922, both parliaments ratified the Treaty, formalising dominion status for the 26-county Irish Free State (which renamed itself Ireland and claimed sovereignty over the entire island in 1937, and declared itself a republic in 1949), while Northern Ireland, gaining Home Rule for itself, remained part of the United Kingdom.

Separate unionist and nationalist historical narratives exist for the historic events in question; nationalist perspectives are further divided by the Civil War which ended the revolutionary period. The Northern Ireland peace process, with its promotion of dialogue and reconciliation, has modified this separation. The Bureau of Military History established by the Irish government in 1947 collected oral history accounts from republican veterans of the period 1913 to 1921. Its records were sealed until the last veteran's death in 2003; they were published online in 2012.