Something that cannot be changed or reversed.

अपरिवर्तनीय

English Usage: The decision was irrevocable and could not be altered.

Hindi Usage: फैसला अपरिवर्तनीय था और इसे बदला नहीं जा सकता था।

Not able to be appealed or revoked.

निषिद्ध

English Usage: Once signed, the contract was irrevocable.

Hindi Usage: एक बार हस्ताक्षर करने पर, अनुबंध निषिद्ध था।