(add-hook 'rcirc-print-hooks 'my-rcirc-print-hook)
(setq alert "/usr/share/sounds/gaim/alert.wav")
(defun my-rcirc-print-hook (process sender response target text)
(when (and (string-match (regexp-quote (rcirc-nick process)) text)
(not (string= (rcirc-nick process) sender))
(not (string= (rcirc-server-name process) sender)))
(start-process "beep" nil "aplay" (expand-file-name alert))))
It is also possible to show libnotify pop-ups with a little Python(link is down).