IRC buffers are
constantly growing. If you want to see as much as possible at all
times, you would want the prompt at the bottom of the window when
possible. The following snippet uses a local value for
scroll-conservatively to achieve this:
(add-hook 'rcirc-mode-hook
(lambda ()
(set (make-local-variable 'scroll-conservatively)
8192)))
See Scrolling conservatively, for details.