If you were to describe something as indescribable, is it really indescribable?

Yes because "indescribable" isn't a description.

Explanation: The exact definition of "description" is "give an account in words of (someone or something), including all the relevant characteristics, qualities, or events."

Therefore the word "indescribable" isn't describing anything. It's just saying that you literally cannot do the things to make it describable (account words of quantities, characteristics, or events)