Irretrievable means something that can’t be retrieved or recovered. If you have irretrievable memory loss, it means you'll never get your memories back.
Golden retrievers are dogs who love to play fetch — they retrieve the ball, or bring it back over and over again. But if the ball gets stuck in a tree, they will just stand there, helplessly waiting for it to fall. Try to explain it a Golden Retriever in Latin: they are not ir- going to be able -able to get -retrieve- that ball = it is ir-retriev-able. If you suffer an irretrievable loss — like the death of someone you are close to — that means a loss you may never recover from.