The Soloheadbeg ambush took place on 21 January 1919, when members of the Irish Volunteers (or Irish Republican Army [IRA]) ambushed Royal Irish Constabulary (RIC) officers who were escorting a consignment of gelignite explosives at Soloheadbeg, County Tipperary. Two RIC officers were killed and their weapons and the explosives were stolen. The Volunteers acted on their own initiative and had not sought authorisation for their action. As it happened on the same day that the revolutionary Irish parliament first met and declared Ireland's independence, it is often seen as the first engagement of the Irish War of Independence.

In April 1916, during the First World War, Irish republicans launched an uprising against British rule in Ireland, called the Easter Rising, where they proclaimed an Irish Republic. After a week of fighting, mostly in Dublin, the rising was put down by British forces. Most of the Rising's leaders were executed. The Rising, the British response, and the British attempt to introduce conscription in Ireland led to even greater public support for Irish republicanism.^{[citation needed]}

In the general election of December 1918, the Irish republican party Sinn Féin won a landslide victory in Ireland, gaining 73 out of 105 seats (25 of these unopposed) in the British Parliament. However, in its election manifesto, the party had vowed to set up a separate government in Ireland rather than sit in the British Parliament. At a meeting in Dublin on 21 January 1919, Sinn Féin established an independent parliament called Dáil Éireann and declared independence from the United Kingdom.

That same day, an ambush would be carried out by Irish Volunteers from the 3rd Tipperary Brigade. It involved Seán Treacy, Dan Breen, Seán Hogan, Séumas Robinson, Tadhg Crowe, Patrick McCormack, Patrick O'Dwyer and Michael Ryan. Robinson (who had participated in the Easter Rising) was the commander of the group that carried out the attack and Treacy (a member of the Irish Republican Brotherhood since 1911) coordinated the planning of the attack. The unit involved acted on its own initiative.

In December 1918, they received information that there were plans to move a consignment of gelignite from Tipperary British Army barracks to the Soloheadbeg quarry. They began plans to intercept the consignment and Dan Breen's brother Lars, who worked at the quarry, received information that the consignment was to be moved around 16 January 1919. They anticipated that there would be between two and six armed escorts, and they discussed different plans. If the escort was small, they believed they could overpower the RIC officers without firing a shot. Gags and ropes were hidden in the quarry, so that if the officers surrendered they could be bound and gagged. The planning for the ambush took place in the 'Tin Hut', a deserted semi-derelict house at Greenane.

Robinson, who had returned to the Brigade area after his release from jail, was briefed by Treacy about the plans to seize the gelignite. Robinson supported the plan and confirmed with Treacy that they would not request permission from the Irish Volunteer leadership. If they did, they would have to wait for a response, and even if the response was affirmative, it might not come until after the gelignite was moved.

Each day from 16 to 21 January, the men chosen for the ambush took up their positions from early in the morning to late afternoon and then spent the night at the deserted house. Seven of the Volunteers were armed with revolvers while Treacy was armed with a small automatic rifle. On 21 January, around noon, Patrick O'Dwyer saw the transport leaving the barracks. The consignment of 160 lb of gelignite was on a horse-drawn cart, led by two council men and guarded by two RIC officers armed with carbine rifles. O'Dwyer cycled quickly to where the ambush party was waiting and informed them. Robinson and O'Dwyer hid about 20 yards in front of the main ambush party of six, in case they rushed through the main ambush position.

When the transport reached the position where the main ambush party was hiding, masked Volunteers stepped out in front of them with their guns drawn and called on the RIC to surrender, shouting "Hands up!" more than once. It was raining. The officers could see at least three of the ambushers; one officer got down behind the cart and the other apparently fumbled with his rifle. According to the Volunteers, the officers raised their rifles to fire at them. Séumas Robinson said the officers attempted to shoot but that the rifles did not fire because "the cut-off had been overlooked". The Volunteers immediately fired at the officers, and it is believed that Treacy fired the first shot. Both officers were killed: James McDonnell and Patrick O'Connell, native Roman Catholics. McDonnell was shot in the left side of the head and through the left arm; O’Connell was shot through the left side, and was likely in a stooping position. McDonnell was born in Belmullet, County Mayo. He was aged 50 at the time of his death and was a widower with five children. O'Connell was unmarried and a native of Coachford, County Cork.