Did SGI publish secure hashes of your IRIX software?
If yes, I bet it's using MD5 at best. Hashes seem to have quite limited lifetimes.
If no, how can you know the system today is as it was before? If you say "cause
it's been sitting in my garage", then how can I repeat your result? Perhaps you
will offer a compiler verification service, but then we're still back to Thompson's