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I don't know if you can see this link Michael, if not its from the Irish Times of June 17 1920. In brief a party of 4 police, whilst arresting a man for a civil offence, were attacked by a stone throwing mob. Constable Pierce Doogue 60412 was killed, HC Thomas Rahill 57694 was severely wounded in the head and Constables Norris and Hanlon were also injured.
http://www.irishtimes.com/newspaper/archive/1920/0617/Pg005.html#Ar00513

Given the elapsed time, its unlikely there was a direct link between Rahill leaving and your grandfather taking over, but you never know. It may be worth looking at Rahill's record.