I highly recommend that you read 'Making Sense of the Troubles' by David McKittrick and David McVea. Although the book focuses primarily on the period 1960-2000, it contains a very comprehensive - and even-handed - introductory chapter which explains the background to Irish partition and independence, including the role of the IRA 1920-22, and the short-lived IRA bombing campaign of the late 1950s, to its early-1960s incarnation as, essentially, a revolutionary socialist outfit. You might also find 'Armed Struggle' by Richard English helpful.