gistfile1.el

(defun erc-notify-on-private-msg-darwin (proc parsed)
  (let ((nick (car (erc-parse-user (erc-response.sender parsed))))
        (target (car (erc-response.command-args parsed)))
        (msg (erc-response.contents parsed)))
    (when (and (erc-current-nick-p target)
               (not (erc-is-message-ctcp-and-not-action-p msg))
               (erc-notify-allowed-p nick))
      (shell-command-to-string
       (format "terminal-notifier -sender org.gnu.Emacs -message '%s says: %s'" nick msg))
      nil)))

(when (eq system-type 'darwin)
  (add-hook 'erc-server-PRIVMSG-functions 'erc-notify-on-private-msg-darwin))