'irreplaceability' originates from Latin and Old French elements via modern English, specifically from the adjective 'irreplaceable' (formed from the negative prefix 'ir-' + 'replaceable'), where 'ir-' (from Latin 'in-' assimilated before 'r') meant 'not' and the root 'replace' comes from Old French 'replacer' meaning 'to place again'.

'irreplaceability' changed from the adjective form 'irreplaceable' (English, formed by adding the suffix '-able' to 'replace') and the noun-forming suffix '-ity', and eventually became the modern English noun 'irreplaceability'. The verb 'replace' itself derives from Old French 'replacer' ('re-' + 'placer').

Initially, the root verb 'replace' conveyed the notion 'to put back or place again', but over time the derived noun 'irreplaceability' evolved to mean 'the state or quality of not being able to be replaced' (a negative evaluation of replaceability).