If youre getting that error, and youre connected to the internet, then that means the (sub)domain youre trying to connect to doesnt exist (or doesnt have an A record to resolve to an IP), the only way to fix this is to basically find another (sub)domain to connect to that goes to the place you want to go to, or by connecting to the ip of the server hosting the ircd directly. A way you can test to see if something resolves is by using the /dns command inside of mIRC. Here is an example:
Google.Com is a domain ...:
[12/25/2006 - 03:10:21] * Dns resolving google.com
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[12/25/2006 - 03:10:21] * Dns resolved google.com to 64.233.187.99
If i were to try to connect to Google.Com, mIRC would try to connect to 64.233.187.99 on port 6667 (mIRC resolves an address to an ip, then tried to connect to the ip it gets. And google.com doesnt host an ircd by the way

Just an example.)
Hope i've helped.