1912–1922

Home Rule became certain when in 1910 the Irish Parliamentary Party (IPP) under John Redmond held the balance of power in the Commons and the third Home Rule Bill was introduced in 1912. Unionist resistance was immediate with the formation of the Ulster Volunteers. In turn the Irish Volunteers were established to oppose them and enforce the introduction of self-government.

In September 1914, just as the First World War broke out, the UK Parliament finally passed the Third Home Rule Act to establish self-government for Ireland, but was suspended for the duration of the war. In order to ensure the implementation of Home Rule after the war, nationalist leaders and the IPP under Redmond supported the British and Allied war effort against the Central Powers. The core of the Irish Volunteers were against this decision, but the majority left to form the National Volunteers who enlisted in Irish regiments of the New British Army, the 10th and 16th (Irish) Divisions, their Northern counterparts in the 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 the Irish sides (Nationalist, Unionist) were unable to agree terms for the temporary or permanent exclusion of Ulster from its provisions.

The period from 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 was made to gain separate independence for Ireland with the 1916 Easter Rising, an insurrection in Dublin. Though support for the insurgents was small, the violence used in its suppression led to a swing in support of the rebels. 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 a majority of three-quarters of all seats in Ireland, twenty-seven MPs of which assembled in Dublin on 21 January 1919, to form a thirty-two county Irish Republic parliament, the first Dáil Éireann unilaterally declaring sovereignty over the entire island.

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 a truce that halted the war. In December 1921, representatives of both governments signed an Anglo-Irish Treaty. The Irish delegation was led by Arthur Griffith and Michael Collins. This abolished the Irish Republic and 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 independence for the twenty-six county Irish Free State (which went on to re-name itself Ireland in 1937 and declare itself a republic in 1949); while the six county Northern Ireland, gaining Home Rule for itself, remained part of the United Kingdom. For most of the next 75 years, each territory was strongly aligned to either Catholic or Protestant ideologies, although this was more marked in the six counties of Northern Ireland.