Complex sentence: It was his actions that initiated the conflict in 1919 but he was killed the following year, in October 1920, in Talbot Street in Dublin, in a shootout with British troops during an aborted British Secret Service surveillance operation.
Split into simpler sentences: It was his actions that initiated the conflict in 1919. He was killed the following year, in October 1920, on Talbot Street in Dublin, in a shootout with British troops during an aborted British Secret Service surveillance operation.