It might be helpful to break the words down. irrevocable = ir + revoc (revoke) + able irrecoverable = ir + recover + able The prefix ir- gives a negative meaning. The suffix -able means "can", "is possible", "is able". In this example, "revoke" means "to change, to reverse". In this example, "recover" means "to find again, to get back". irrevocable means "to not be able to revoke (reverse/change)". The decision was irrevocable. irrecoverable means "to not be able to recover (find again, get back)". If you drop a pin in the ocean, it is irrecoverable.

http://www.thefreedictionary.com/irrevocable http://www.thefreedictionary.com/irrecoverable You revoke a decision. You recover something you gave away.