On Mac OS X or Linux you can use Ctrl + L to clear the IRB screen.
Throw this inside %userprofile%\.irbrc and you're good
%userprofile%\.irbrc
def cls system('cls')end
From IRB clear screen on windows.
On *nix boxes
`clear`
on Windows
system 'cls' # works`cls` # does not work
on OSX
system 'clear' # works`clear` # does not work