Presumably Irene was collecting her data before she ever met Sherlock...so "I am Sherlocked" couldn't always have been her password.
Granted, there's no great mystery about this: people change passwords all the time. But he even points out, if she'd chosen something more random, he might not have figured it out. Well, all she really had to do was just leave it the way it was before she was "sherlocked."