From my little experience with jenkins, I haven't met any other plugin which requires me to do special setup with system properties to change a setting for a plugin.

It gives a fair and clear warning that the user shouldn't change it if the user doesn't know what it means. The trust all certificates option is more «dangerous», as that is simple the same as using a plain connection.  

Anyhow:
I guess you could have a help text and referrer them to set a system property hudson.plugins.irc_publisher.messageRate (?) and give a hint it can be set in «Configure jenkins - Environment variables» .. but sounds complicated and unfriendly.