When you connect to IRC with mIRC you can use the /server command to connect. See /help /server.

Example: /server irc.dal.net 7000

irc.dal.net is the server, and 7000 is the port. You can change servers by replacing irc.dal.net with another server name, for example, irc.quakenet.net or irc.undernet.org, or an individual server of a particular network - the latter server names are "round robins", meaning they are a generic server name that forward you to a specific server on that network. Individual servers are usually listed on the network's website (example) - so you might do /server mesra.dal.net 7000 to connect to DALnet.

The default port mIRC uses is 6667 - the vast majority of IRC servers support this port. However, sometimes it may be easier to connect on another port, usually 6668, 6669, 7000 or 7001, though it could be a whole host of others. If you have no luck after changing the port 3-4 times, it's likely that it's not going to help at all.

Does that help at all?