Why am I doing the job for Speechtouch tech support?
Anyway firstly you need to specify the DCC port range in your IRC client, if you use X-chat this would be in "Setting - Preferences - Network - File Transfers" just set it to something like 9000-9005(or whatever range you like, but make it >1024 and no conflict with other services).
Now go to the web interface for your Speedtouch route(something like http://10.0.0.138
by default according to the manual)
select Advanced - NAPT and in NAPT Entries click add new entry
for the outside address leave it unspecified, tcp, then 9000 on the port
for inside address set the ip to the local ip of your machine(/sbin/ifconfig) and the port on 9000 as well, repeat this for port 9001-9005. Then you probably need to reboot the router, not sure, and that should be it. Also if you have a firewall running on the local machine make sure it isnt blocking port 9000-9005.