You likely need to set up port forwarding on your router,
this thread may be of some help, as well as the
technical support section for Westell (ick, yukky link, if that fails, just navigate from
www.westell.com).
You will also need to give mIRC permissions to 'act as a server' (or similar) in zone alarm.
Hope that helps
[edit: removed some parameters from the westell link]