[RayW]

You may have to edit the iptables rules file manually. For CentOS, I believe that is located in /etc/sysconfig/iptables - What you'll need to do is place the ACCEPT rules above all the DROP and REJECT rules... OR you could flush iptables complete (iptables -F) and re-add all your iptables one by one, starting with the ACCEPT rules for IRC.

If that still does not work, can you provide a copy of iptables -L ?