I discovered recently that net-im/finch can indeed join password protected IRC channels even though the channel add dialogue box doesn’t support it.
Add an IRC chat room that requires a password to your buddy list.
Exit out of finch and edit ~/.purple/blist.xml
Find the chat room you just added:
<chat proto=’prpl-irc’ account=’firstname.lastname@example.org’>
<setting name=’gnt-autojoin’ type=’bool’>1</setting>
Simply add the password line after the channel name component:
Restart finch and you will now be able to auto-join the IRC channel.
Note that the password for this IRC channel is now in PLAIN TEXT in a file in your home directory. So, ensure that you are doing this somewhere trustworthy.