'irrevocableness' derives from the adjective 'irrevocable', which comes from Latin 'irrevocabilis' (in- 'not' + 'revocare' 'to call back'), with the nominalizing suffix '-ness' added in English to form a noun meaning 'the state of being not able to be revoked'.

'irrevocabilis' in Latin passed into Anglo-Norman/Old French as forms like 'irrevocable' and entered Middle English as 'irrevocable'; English then formed the noun by adding the native suffix '-ness' to create 'irrevocableness'.

Originally it meant 'not able to be called back' (literally 'not called back'), and over time it has come to mean more generally 'the state or quality of not being able to be revoked, undone, or changed'.