On Easter Monday, 24 April 1916, Irish nationalists along with some 1.600 followers seized government buildings in Dublin seeking the ouster of Britain from Ireland. It was planned by Patrick Pearse, Tom Clarke and others in the Irish Republican Brotherhood and was part of a larger group called Irish Volunteers. The Irish Volunteers had around 16,000 members and had obtained German weapons in 1914. There was also the Irish Citizen Army which was comprised of Dublin workers who had been of the failed general strike in 1913. The small Sinn Fein party was also involved as well.
Though initially planned to be national in scope but ended up primarily in Dublin. The British had learned of the uprising and had made several arrests on April 21. Patrick Pearse and Tom Clarke went ahead with their plans and seized the Dublin General Post Office and other points in the Dublin City Center. Pearse read a proclamation announcing the birth of the Irish nation. British troops were sent in to put down the rebellion, which began a week-long battle in the streets between the two. Dublin was paralyzed during this time. British artillery was used against the rebels and forced their capitulation in the end.
All of the leaders of the Easter Rising were court-martialed and executed. The uprising itself was not generally supported by most Irish. However, the executions and arrest of anyone thought to have been involved with the rebellion angered many along with the imposition of martial law turned the leaders into martyrs. Instead of stamping out the rebellion, the British response only fueled the cause for many to want an Irish free state (sounds familiar to those who study the American Revolution).
In 1919, the Irish Republican Army began a guerilla war against the British in Ireland. A cease fire was called in July 1921 and a treaty was signed in 1921. The treaty established the Irish Free State as a self-governing nation of the British commonwealth. However the treaty allowed for an opt-out provision and the six northern counties decided to stay with the United Kingdom. This was not wholly popular with many in the Irish leadership (and led to internal strife and killings). It was not until Easter Monday in 1949 that the Republic of Ireland was fully declared (except for the northern counties that had opted out).