This thread is a good one to read when it comes to connection issues, it explains a lot.
The most likely explanation is that that particular address is closed off - it could be for a number of reasons. If you know any individual server address then you might try connecting to those. Also look at the network's website, if you don't know it, try
Google.
Also try using other ports. I tried it out with several and none worked, but try some in the 7000 > range.
It could be that the network is just down...try again in 12-24 hours, hopefully it will be up. However, this is not down to your connection or mIRC.
Hope you get on soon
Regards,