Most of the Nationalist IVF did go to war alongside the British. However a small splinter group disagreed with this policy of helping the British and stayed at home. In order to disassociate themselves, the majority of the IVF renamed themselves the National Volunteer Force (NVF) while the splinter group remained the IVF. Thousands of Irishmen joined the war, and these men became the British Army's 10th and 16th divisions.

Many of the UVF men also joined the war, along with other Unionists. These men became the 36th Ulster Division. On 1st July 1916, in France, the 10,000-strong 36th Ulster Division took part in a major offensive known as the Battle of the Somme. This offensive turned out to be one of the worst military routs of the war, and there were 5,000 casualties among the 36th Ulster division alone. London viewed this sacrifice, on the part of the men of Ulster, as an indication that Ulster could not now be forced into Home Rule.

When the war had begun in 1914, the government had told troops that they would be 'home by Christmas' (in other words that the war would be over by the end of 1914). By 1916 the war was still at a stalemate, and Nationalists began to realise that the war could go on for years. So the Irish Republican Brotherhood and the splinter IVF planned a huge rebellion to drive the British out of Ireland, taking advantage of the fact that the British had few troops to spare. It was led by a Dubliner, Patrick Pearse, along with Thomas MacDonagh and Joseph Plunkett. The rising was planned for Easter 1916, and was to be supplied with German weapons by Roger Casement. Despite the fact that the weapons were captured by the British, the rebellion went ahead on Easter Monday (24 April) 1916.

1,500 rebels took over the Dublin Post Office and other key buildings in the city. They then raised the Irish Flag and read a proclamation of independence and formation of the Republic of Ireland. A fierce battle ensued between the rebels and the British. On 29 April, after 5 days of mortars, shells and gunfire, the rebels surrendered after 450 volunteers had been killed. Huge areas of Dublin city centre were in ruins and many locals sided with the British and shouted abuse as the rebels were lead away. Their opinions changed, however, when it was announced that the leaders should be executed for treason and collaboration with the enemy (Germany). Almost 100 men were shot after nominal trials. The British wrongly blamed Sinn Féin for the rising (it had actually been the Irish Republican Brotherhood) and this contributed greatly to the Home Rule Party's defeats and Sinn Féin's success in the next election.

In July 1917, Eamonn de Valera became the President of Sinn Fein. He had taken part in the Easter Rising, but had not been executed. He stood in the Clare East by-election, openly declaring his belief in an Independent Irish Republic. He won easily, but refused to take up his seat at Westminster as part of Sinn Fein's policy of abstention. The Sinn Fein candidates became popular because the British had blamed them incorrectly for the Easter Rising and had then executed the leaders of the rising. This provoked much sympathy towards Sinn Fein amongst Irish voters, and Sinn Fein did not attempt to set the record straight.

Sinn Féin gained even more support when they led the successful fight to prevent conscription in Ireland to feed the First World War trenches in 1918. After the war, which ended with German defeat in 1918, Sinn Féin won 73 seats compared to the Home Rule Party's 6. The Irish Unionist Party won 26 seats, mostly in Ulster. All 73 Sinn Féin MPs refused to go to Westminster, and instead sat in their own Parliament in Dublin. Called Dail Eireann, it first met on 21st January 1919, although it had no power to exercise.