MacBook not able to connect to IRC server over Airport
When you network the Mac directly into the novuscom device (bypassing the Aiport) are you able to connect to your IRC?

Is the Airport Express acting as a DHCP server?  Is the novuscom device also acting as a DHCP server?  Problem might be the double NAT.

If so does putting the Airport Express into bridge mode fix the problem?

This discussion also looks promising:

Apple's routers do not use UPnP.

They do offer a similar type of feature, which is called NAT-PMP, or NAT Port Mapping Protocol

Open AirPort Utility - Click Manual Setup
Click the Internet icon
Click the NAT tab below the icons
Insure that there is a check mark next to Enable NAT Port Mapping Protocol

Click the Configure Port Mappings button if you need to specify specific ports to use and set them up on the next window that appears