Is it possible to figure out whether or not some user is connected through SSL on an IRC server? I want to figure out a way to restrict individuals who don't connect that way (from joining a specific channel).
I'd want the access to be from the level of a bot or individual.
You'll need to talk to the server operator and see if they'll add a user mode bit to signify SSL connection (like the "registered nick" and "auth to nickserv" bits most servers have). I'm pretty sure there's no "standard" way to determine this information.
This depends completely on the IRC server you're running. On UnrealIRCd, the +z channel mode automatically prevents non-SSL users from joining the channel. (For users, the corresponding +z user mode indicates an SSL connection.)
/whois NICK to see what kind of connection they are using. if you see "$nick is using a secure connection", then they are using SSL.
As per your second question, most standardly the two modes to toggle on SSL only mode is +z, as the above user said, or, on some servers +S. Note that's a big S not small s. Small s is for "secret", or, unlisted, mode, which is nice to have, but totally different.
Happy irc-ing! :D