you have to load the ipable modules for dcc. Be sure you have allowed port forwarding and tell your firewall about dnat.
ip_conntrack_irc and ip_nat_irc
you'll probably also have to add a a line in your firewall script using the state table. it should look like
iptables -A INPUT -i YourInternetInterface -p tcp -m state --state RELATED,ESTABLISHED -j ACCEPT
( not being at home i can't give you the exact line. it may also depend of how tight is your firewall)
i had also to set mirc to use my lan ip and not the internet ip of the linux box
see
iptable tutorials