> So, is there a way to save a set of irb commands that I just typed to
> a file, so they can be rerun later?

You can save the history of irb commands. See:
http://rubygarden.org/ruby?Irb/TipsAndTricks

> Better yet, I'd like to save the
> results of the commands too, and check to see if any results have
> changed on a re-run.

irb | tee log
If you're on *nix/cygwin.

HTH,
Assaph