What is ironic about the phrase "the mutability of the past"?after mentioning the constant surveillance under which the people live, winston says...

First of all, the one thing that is your own is the "the few cubic centimetres inside your skull."

As far as the first answer goes, I agree that you are probably expected to say that the quote is ironic because the past is immutable.  However, I disagree with the idea that the past truly is immutable.

We do not really know anything about the past except what we are told.  Therefore, when what we are told changes the past, in essence, changes.  The study of history is NOT a study of facts and figures.  It is the study of WHY things happened in the past and what that can tell us about today.

The stories about why things happen change over time.  Many things in textbooks today are very different from what used to be in textbooks.  So I'd argue the past is mutable.

But that's not to say the previous answer is wrong.  It's just to say that I disagree with the question.