You need to register an account with those networks, then login and then use /mode your-nickname +x where a default network vhost will be set on you. This hides your IP address from normal users but IRC Operators will still be able to see your IP in your /whois.
So, you register an account on the network that you are at.
If you don't know how to do this, go to the networks helpchannel, usually #help
Then you go to your perform menu in mIRC:
Tools - Options - Connect - Options - Perform
Click "Enable perform on connect"
In the square below that, you type the login line for your network (the login you just created, after visiting the help channel).
Below that line you type:
/mode $me +x
That should make your IP invisible, everytime you connect.