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.