The Irish War of Independence (or the Anglo-Irish War) was a guerrilla campaign mounted against the British government in Ireland by the Irish Republican Army under the proclaimed legitimacy of the First Dáil, the extra-legal Irish parliament created in 1918 by a majority of Irish MPs. It lasted from January 1919 until the truce in July 1921.
To purist Irish Republicans, the Irish War of Independence had begun with the Proclamation of the Irish Republic during the Easter Rising of 1916. Republicans argued that the conflict of 1919-21 (and indeed the subsequent Irish Civil War) was the defence of this Republic against attempts to destroy it. More directly, it had its origins in the formation of an unilaterally declared independent Irish parliament, called Dáil Éireann, formed by the majority of MPs elected in Irish constituencies in the Irish (UK) general election, 1918. This parliament, known as the First Dáil, and its ministry, called the Aireacht, declared Irish independence.

The Irish Republican Army which fought in this conflict is often referred to as the Old IRA to distinguish it from later organisations that used the same name. Volunteers began to attack British government property, carried out raids for arms and funds and targeted and killed prominent members of the British administration.

They mimicked the successful tactics of the Boers, fast violent raids without uniform. Although some republican leaders, notably Éamon de Valera, favoured classic conventional warfare in order to legitimise the new republic in the eyes of the world, the more practically experienced Michael Collins and the broader IRA leadership opposed these tactics. The violence used was at first deeply unpopular with the broader Irish population, but most were won around when faced with the terror of the British government's campaign (response) of widespread brutality, destruction of property, random arrests and unprovoked shootings. Events began slowly, but by 1920 widespread violence was the rule.

Custom House in Dublin

In May 1921, IRA units occupied and burned the Custom House in Dublin city centre. Symbolically, this was intended to show that British rule in Ireland was untenable.  Many military historians have concluded that the IRA fought a largely successful and lethal guerrilla war against Britain, which concluded that the IRA could not be defeated militarily. The total numbers killed in the guerrilla war between Republicans and Crown Forces of 1919-21 came to over 1,400.
The war ended in a Truce on July 11, 1921, in some respects, the conflict was at a stalemate.  Ultimately, the peace talks led to the negotiation of the Anglo-Irish Treaty (1921).
 

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1. The text was extracted from Wikipedia The Free Encyclopedia under GNU Free Documentation Licence.