To understand the irony behind the quote (from George Orwell's novel 1984), "the mutability of the past," one must first understand what the terms being used mean.
Mutability is defined in the following way:
Capable of or subject to change or alteration.
The irony lies in the fact that history is just that, history (events from the past). History, given that it has already happened, is not able to be changed, it is certainly not "subject to change or alteration." Therefore, history is not capable of being undone or being altered for the fact that it has already happened.The irony is found in the fact that someone, by saying that history is mutable, believes that history can be altered--when it cannot.
Another way one can look at the irony in the statement is the fact that some people believe that history can be altered. While history cannot literally be altered, the way one interprets history can be. Therefore, one who thinks that history can be altered is, basically, not looking at history as something from the past. Instead, they are looking at history as something which exists in the present.