In a nutshell, what events, occurrences, or mishaps took place that allowed the Irish to win the war of Independence over what could arguably be the most powerful nation of the time, when they were unsuccessful in their multitude of attempts previously?
World War I is the short answer. Conscription efforts incensed large volumes of the Irish population and the massive casualties sustained as well as economic damage from fighting the war made it harder for the British to respond. I'm sure the 1918 flu didn't help matters any.
One of the factors of Irish independence in 1918 was U.S. President Wilson's Fourteen Points declaration, which included national self-determination. Used as a weapon against Germany and her allies in favor of central European peoples, it was also held against the British in favor of the Irish.
Also, Britain had been weakened and sickened by the carnage of World War I. It did not have the stomach for another one, even a "small" war.
The one other time an Irish independence movement received outside help was in 1798 from 1100 French soldiers in "the year of the French." But that was too little aid to secure the country's independence.
The Irish War of Independence is usually viewed at starting in 1916, with the Easter Rising. It was militarily a disaster, since they surrendered after a few days. The British government executed most of the the ring leaders, and turned them into martyrs, further helping the Irish Independence movement.
The British were exhausted, and indeed bled dry by WWI. There was no will for yet another war of attrition, and also, Michael Collins was a master at guerilla war.