This is from a previous answer of mine on the topic:

The influence of Michael Collins and the pre-Irish Civil War IRA on post-1920s guerrilla campaigns is low and reached its peak in the late 1940s. Yitzhak Shamir, the leader of the Stern gang (or more officially, Lehi) and Vladimir Jabotinsky (who founded Irgun) were both influenced by Michael Collins in their campaigns against the British. However, this made perfect sense for Shamir and Jabotinsky (and, without any solid confirmation, Menachem Begin) since he too was fighting a war of anti-British colonialism. Shamir and Jabotinsky are not the only examples. Subhas Chandra Bose urged his followers to study the IRA. Irish texts were even translated into Burmese during the 1930s and were read by men like Ba Maw.

The truth of the matter is that there was nothing that particularly special or groundbreaking about the campaign carried out by the IRA. Collins himself was a student of and clearly influenced by the Boer commandos during the Second Boer War (1899-1902) and in particularly of the Boer general Christiaan de Wet, which he based the IRA flying columns on. There are some who claim that he 'invented' urban guerrilla warfare, overlooking the fact that theories had been printed on this since the late 19th century. The main reason for his influence, as we can see above, is that he fought a relatively successful guerrilla war against the British and was involved in anti-British activities. This clearly brought him and his methods to the attention of those also looking to combat the British, but certainly not all of them. It is interesting to note that the guerrilla campaigns during the Irish War of Independence also influenced the creation of the Special Operation Executive during WWII, which makes sense since this would have been the most accessible and recent guerrilla warfare campaign to be studied by the British.

By the 1950s, his influence appears to have vanished as insurgents the world over searched for and adapted to new theories. These theories, ranging from Mao Zedong's theory on guerilla warfare to the Guevara Foco theory, were only the basics for insurgents who adapted their strategies to a world of mass media and international discourses that the IRA of the late 1910s and early 1920s wouldn't have been able to predict.