There are no special settings required to run an IRC client . Even DCC (Direct Chat/Send(Receive) Files) will work automatically if you use the standard port 6667.
To run the IRC server behind NAT please map the following ports:
Protocol: TCP
Listen IP: unspecified or IP you want to use for IRC server
Listen Port: 6667
Destination IP: IP address of PC with your IRC server
Destination Port: 6667
Using anything other than the standard port will cause DCC to not work.