Guess what? The very few Regular RIC who did break the law seem to have got away with it as did the 'Tans, Auxiliaries, USC and British Army. Move on, it is history and they are all dead now.I think you are mistaken, English Common Law, which applied in Ireland at that time, cannot simply be overturned by a ruling of the cabinet.
If it is unlawful for a police constable to loot a public house or shoot the Lord Mayor of a large town then according to the law to do so is a crime.
The cabinet in 10 Downing Street deciding that in certain circumstances it might be acceptable to do so if the victims are Irish does not over rule Common Law.