Thats not an easy question to answer, we dont know what router you have, if you have a firewall, etc. Basically you need to be sure you arent blocking the server ports (6660-6669, 7000-7001 and 113) or mIRC.
Usually when you get unable to resolve server it usually means that server is down/doesnt exist or you are having a dns prob. Have you tried the following:
/server irc.Prison.NET
/server 208.178.231.190