the word you are looking for here is ‘regime change’. it is true that it is unreasonable to bring up ‘historic matter’, that is conduct in the ‘prior regime’, to discuss the merits of the current regime. Germany suffered total defeat in WW2 and so its “1944” prison conditions mean nothing. UK arguably can claim “a win” after the end of USSR (the only possible ‘regime change’ we can consider as relevant matter) and so ‘regime vindicated!’ and thus conduct of UK against Irish resistance is relevant as evidence.