I wouldn't say Irish independence was gotten easily, especially when you consider that there were defeated rebellions in 1798, 1803, 1848, 1867 and 1916, though total dead for the Irish War of Independence from 1919 to 1921 is just 2,000 which puts it firmly at the lower end of the violent struggles at the time.

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Britain wasn't able to and didn't want to deploy its full military power in Ireland for a number of reasons. Ireland was a part of the United Kingdom and to do so would have been an admittance that they were fighting a civil war. The large Irish-American population in the United States, in retaliation for Woodrow Wilson's lack of support at the Paris Peace Conference, helped block US entry into the League of Nations and abstained or voted for Republican Warren G. Harding in the 1920 Presidential Elections. They were constantly bringing resolutions to Congress and highlighting British atrocities in Ireland after the deployment of the Black and Tans in 1920. Irish demands for independence had considerable sympathy in the US and Britain couldn't risk using military force.

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Not that they didn't try. Ex-British soldiers demobbed from the army after the First World War were given military equipment, partial military attire and assigned to the police, as the War of Independence was only a policing action dealing with criminals in Britain's eyes. The Black and Tans as they came to be known due to the mix of police and military uniform were ill disciplined and carried out reprisals against the civilian population, further alienating them from Britain and increasing support for the Irish Republicans. On the 11th of December 1920 they sacked and burnt Cork City. Their actions were publicized not alone by Irish Republicans but also by British newspapers which caused backlash at home against the government.

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Without deploying the entire army and imposing martial law Britain was unable to deliver a killer blow that would have knocked out the Irish Republican Army. By early 1921 the IRA had formed Flying Columns which took to the mountains, lived off the land and local support and carried out attacks against isolated patrols in localities they had grown up in. The IRA could pick the best ambush points and could use the local people for information and scouting. During the Troubles in Northern Ireland the British Military would make the same assessment; the IRA had no chance on winning but they also could not be beaten. Peace was suggested in late 1920 which the Irish responded enthusiastically to. In fact they responded so enthusiastically that the British thought they were on the verge of defeat and redoubled efforts to destroy the IRA. The IRA was able to respond and in turn escalated its campaign to show it could keep going. In March 1921 at Crossbarry, Co. Cork, 100 IRA soldiers attacked a 1,200 strong British column. The British suffered only 10 soldiers killed, with 6 casualties on the IRA side, but a few days later in Killarney Co. Kerry 20 British troops were killed to 2 IRA losses. Each soldier lost by the IRA was devastating, by this stage they were running low on man power and equipment but they were able to show that they were not beaten.

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Finally, even if the British had been able to deploy its full forces and bring the country to heel it would only been for a short while. Suppressing Irish demands for independence had only kicked the problem a few years or decades down the road. Britain was not prepared to offer full independence to Ireland but went as far as Dominion Status. Britain could now paint itself as the accommodating party, offering Ireland a compromise that would give them control over their own affairs. If Ireland rejected this Britain would then have the justification to use additional military force without suffering backlash from the United States. Narrowly, the compromise was accepted but led to a split in Sinn Féin and the Irish Civil War. 6 of the 32 counties of Ireland are also still part of the United Kingdom and it would be 1949 before the 26 counties split off fully from Britain.

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Ireland was able to win independence between 1919 and 1921 with only a handful of active combatants and even less weapons and ammunition by waging a successful international propaganda campaign, using Irish-America to collect millions of dollars and convincing the British that it would be more costly to suppress the revolution than to negotiate with it. These are some of the key factors, there is a ton of other stuff like the intelligence war carried out mainly in Dublin. This again played into the idea of victory for either side being impossible.