The guerrilla phase of the Irish Civil War lasted another 10 months, before the IRA leadership issued a "dump arms" order to all units, effectively ending the conflict.

The Treaty also allowed Northern Ireland (the six north-eastern counties – Fermanagh, Antrim, Tyrone, Londonderry, Armagh and Down – where collectively the majority population was of the Protestant religion)[5] to opt out of the new state and return to the United Kingdom – which it did immediately.

[7] The treaty also stipulated that members of the new Irish Oireachtas (parliament) would have to take the following "Oath of Allegiance": I... do solemnly swear true faith and allegiance to the Constitution of the Irish Free State as by law established, and that I will be faithful to His Majesty King George V, his heirs and successors by law in virtue of the common citizenship of Ireland with Great Britain and her adherence to and membership of the group of nations forming the British Commonwealth of nations.

In contrast, the Minister of Defence, Richard Mulcahy, stated in the Dáil on 28 April that conditions in Dublin had prevented a Convention from being held, but that delegates had been selected and voted by ballot to accept the Oath.

Arthur Griffith was in favour of using force against these men immediately, but Michael Collins, who wanted at all costs to avoid civil war, left the Four Courts garrison alone until late June 1922.

This completely undermined the electoral pact between the pro- and anti-treaty factions, who went into the Irish general election on 16 June 1922 as hostile parties in some constituencies, both calling themselves Sinn Féin.

[32] Meanwhile, under the leadership of Michael Collins and Arthur Griffith, the pro-treaty Provisional Government set about establishing the Irish Free State, and organised the National Army – to replace the IRA – and a new police force.

In practice, this meant that by the summer of 1922, the Provisional Government of Southern Ireland controlled only Dublin and some other areas like County Longford where the IRA units supported the treaty.

Fighting ultimately broke out when the Provisional Government tried to assert its authority over well-armed and intransigent Anti-Treaty IRA units around the country – particularly a hardliner group in Dublin.

[33][34][35][disputed – discuss] Winston Churchill assumed that the Anti-Treaty IRA were responsible for the shooting and warned Collins that he would use British troops to attack the Four Courts unless the Provisional Government took action.

[37] On 26 June anti-treaty forces occupying the Four Courts kidnapped JJ "Ginger" O'Connell, a general in the National Army, in retaliation for the arrest of Leo Henderson.

[40] Collins ordered Mulcahy to accept a British offer of two 18-pounder field artillery for use by the new army of the Free State, though General Macready gave just 200 shells of the 10,000 he had in store at Richmond barracks in Inchicore.

[41] Historians dispute whether the PRO was intentionally destroyed by mines laid by the Republicans on their evacuation, or whether the explosions occurred when their ammunition store was accidentally ignited by the bombardment.

When the fighting in Dublin died down, the Free State government was left firmly in control of the Irish capital and the anti-treaty forces dispersed around the country, mainly to the south and west.

The Anti-Treaty IRA claimed that it was defending the Irish Republic declared in 1916 during the Easter Rising, confirmed by the First Dáil and invalidly set aside by those who accepted the compromise of the Free State.

There were also a significant number of former members of the British Armed Forces on the Republican side, including such senior figures as Tom Barry, David Robinson and Erskine Childers.

Sporadic fighting also took place around Dundalk, where Frank Aiken and the Fourth Northern Division of the Irish Republican Army were based, and Dublin, where small-scale but regular attacks were mounted on Free State troops.

[64] In the autumn and winter of 1922, Free State forces broke up many of the larger Republican guerrilla units – in Sligo, Meath and Connemara in the west, for example, and in much of Dublin city.

[69] The fictitious "Acts" nevertheless play an important role in the Irish state's foundation myth, dividing civil war violence into bogus legal and illegal categories.

The next day four prominent Republicans held since the first week of the war — Rory O'Connor, Liam Mellows, Richard Barrett and Joe McKelvey — were executed in revenge for the killing of Hales.

The most notorious example of this occurred at Ballyseedy, where nine Republican prisoners were tied to a landmine, which was detonated, killing eight and only leaving one, Stephen Fuller, who was blown clear by the blast, to escape.

The Catholic Church also supported the Free State, deeming it the lawful government of the country, denouncing the IRA and refusing to administer the Sacraments to anti-treaty fighters.

It is often suggested by historians including Professor Michael Laffan of University College Dublin,[84] that the death of Lynch allowed the more pragmatic Frank Aiken, who took over as IRA Chief of Staff, to call a halt to what seemed a futile struggle.

[87] The High Court of Justice in Ireland ruled on 31 July 1923 that a state of war no longer existed, and consequently the internment of Republicans, permitted under common law only in wartime, was now illegal.

[88] Without a formal peace, holding 13,000 prisoners and worried that fighting could break out again at any time, the government enacted two Public Safety (Emergency Powers) Acts on 1 and 3 August 1923, to permit continued internment and other measures.

[88][89][90] Thousands of Anti-Treaty IRA members (including de Valera on 15 August) were arrested by the Free State forces in the weeks and months after the end of the war, when they had dumped their arms and returned home.

In addition, their subsequent guerrilla campaign caused much destruction, and the economy of the Free State suffered a hard blow in the earliest days of its existence, as a result.

This adversely affected the boundary negotiations in 1924–25, in which the Free State government acquiesced that border with Northern Ireland would remain unchanged in exchange for forgiveness of the Imperial debt.

The cost of the war and the budget deficit it caused was a difficulty for the new Free State and affected the Boundary Commission negotiations of 1925, which were to determine the border with Northern Ireland.

Civil War veterans include Republicans Éamon de Valera, Frank Aiken, Todd Andrews and Seán Lemass; and Free State supporters W. T. Cosgrave, Richard Mulcahy and Kevin O'Higgins.