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=’you@irc.server.net’>
<component name=’channel’>#supersekritircchannel</component>
<setting name=’gnt-autojoin’ type=’bool’>1</setting>
</chat>

Simply add the password line after the channel name component:

<component name=’password’>supersekritpass</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.