If you can't undo something, it's irreversible. Time is always irreversible, as is the damage done to your skin by too much sun, so be sure to wear sunblock!
While you see reverse inside of this word, you wouldn't use it to describe a car that can't go backwards. However, if the car was in an accident and cannot be fixed, you could say that the damage done to the car was pretty much irreversible, in other words the car is totaled.
