Restricting the users which can take part in a session is quite easy: you can make your channel password protected (in IRC this password is called a key).

Unfortunately the IRC plugin currently cannot do advanced IRC commands. But this is no problem, simple use another IRC client which is capable of "setting the modes +k, +p and +s", and let it run in the background when you run your session.

Connect to the IRC server with this advanced IRC client and create a channel with the name of your choice (simple join a channel that does not already exist). Then give the channel a key (this is the password, IRC mode "+k <key>"), but please be careful when choosing it, since everyone can see it (most of the clients show the password in plain text, not in asterisks!). Now if you want to join that channel, you will have to pass the password as last parameter of your join command. This works with the IRC plugin, so please do the following in the IRC plugin:

/connect your.irc.server.com
/join #channel <key>

Replace the parameters with the appropriate values. Note: you type in the key in plain text!

To make the protection even better, make the channel private and secret (that is +p and +s). A private channel is not shown in a /whois list (a list which shows the channels someone is joined in) and a secret channel is not shown in the global channel list.