YourBNC mIRC

Open up mIRC, and navigate to the server list by pressing the options icon on the toolbar and selecting Servers under the Connect category.
http://i.imgur.com/pFnjUbh.png

To add a new network to the server list, press the Add button on the right hand side, so the Add Server dialog appears.
http://i.imgur.com/o3jXakG.png

In the dialog, set the description to something that describes the server. In my client, I set it to the name of the network, followed by BNC.
For the irc server box, type in the address of the BNC server. It will be in the format servername.yourbnc.co.uk .
In the ports box, type in +2223 if you want to use SSL, or 2222 if not. If you are using an older version of mIRC, you'll need to install OpenSSL from http://www.mirc.com/ssl.html for SSL to work.
In the groups box, I would recommend setting it to network name(BNC), so the server can be distinguished.
And in the Password box, type in the server password. The server password uses the format username/network:password
For Example, if my username is Hobbyboy, the network is EsperNet and the password is p4ssw0rd, the server password would be: Hobbyboy/EsperNet:p4ssw0rd
http://i.imgur.com/Ir2pUHH.png

To connect to the newly added server, double click on the server in the server list, and press the connect button.

If you want mIRC to automatically connect to the BNC, or several networks on startup, open up the Scripts editor using the button on the toolbar, or by using ALT+R.
When in the scripts editor, navigate to the remote tab, and create a new file using File, New.
In the new script, add the following code:

on *:START:{
  server -poc Network1(BNC)
  server -mpoc Network2(BNC)
  server -mpoc Network3(BNC)
}

Then save the file, by using File, Save.
If you want to automatically join channels specifed in another script or in perform, you can ommit p and c flags from the server commands in the above script.