I have just installed Jenkins 1.620, and am running it with security off. I have installed the IRC Plugin (2.26), and get an exception thrown whenever I send commands to the bot in IRC, like

!jenkins help

It looks like in Bot.java, around line 134, this.authentication returns non-null, and throws a fault when used with security off. The stack trace:

Jul 19, 2015 11:32:31 PM WARNING hudson.plugins.im.bot.Bot onMessage
org.acegisecurity.userdetails.UsernameNotFoundException:
at hudson.security.SecurityRealm$None$2.loadUserByUsername(SecurityRealm.java:494)
at jenkins.security.ImpersonatingUserDetailsService.loadUserByUsername(ImpersonatingUserDetailsService.java:32)
at hudson.model.User.impersonate(User.java:309)
at hudson.plugins.im.IMConnectionProvider$1.getAuthentication(IMConnectionProvider.java:128)
at hudson.plugins.im.bot.Bot.onMessage(Bot.java:136)
at hudson.plugins.ircbot.v2.PircListener.onMessage(PircListener.java:79)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.pircbotx.hooks.ListenerAdapter.onEvent(ListenerAdapter.java:98)
at org.pircbotx.hooks.managers.ThreadedListenerManager$1.run(ThreadedListenerManager.java:110)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

Relevant config:
<useSecurity>false</useSecurity>
<authorizationStrategy class="hudson.security.AuthorizationStrategy$Unsecured"/>
<securityRealm class="hudson.security.SecurityRealm$None"/>