After updating from 1.30 to 1.35, I encountered the following stack trace:

{{SEVERE: Failed Loading plugin ircbot
Throwable occurred: java.io.IOException: Unable to load hudson.plugins.ircbot.PluginImpl from ircbot
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:422)
at hudson.PluginManager$2$1$1.run(PluginManager.java:371)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:169)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:282)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:905)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:210)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:908)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:931)
at java.lang.Thread.run(Thread.java:738)
Caused by: java.lang.UnsupportedClassVersionError: JVMCFRE003 bad major version; class=hudson/plugins/im/IMPlugin, offset=6
at java.lang.ClassLoader.defineClassImpl(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:275)
at jenkins.util.AntClassLoader.defineClassFromData(AntClassLoader.java:1139)
at hudson.ClassicPluginStrategy$AntClassLoader2.defineClassFromData(ClassicPluginStrategy.java:799)
at jenkins.util.AntClassLoader.getClassFromStream(AntClassLoader.java:1310)
at jenkins.util.AntClassLoader.findClassInComponents(AntClassLoader.java:1366)
at jenkins.util.AntClassLoader.findClass(AntClassLoader.java:1326)
at sun.reflect.GeneratedMethodAccessor7.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:37)
at java.lang.reflect.Method.invoke(Method.java:611)
at jenkins.ClassLoaderReflectionToolkit.invoke(ClassLoaderReflectionToolkit.java:44)
at jenkins.ClassLoaderReflectionToolkit._findClass(ClassLoaderReflectionToolkit.java:86)
at hudson.ClassicPluginStrategy$DependencyClassLoader.findClass(ClassicPluginStrategy.java:677)
at java.lang.ClassLoader.loadClass(ClassLoader.java:660)
at java.lang.ClassLoader.loadClass(ClassLoader.java:626)
at jenkins.util.AntClassLoader.findBaseClass(AntClassLoader.java:1398)
at jenkins.util.AntClassLoader.loadClass(AntClassLoader.java:1075)
at java.lang.ClassLoader.loadClass(ClassLoader.java:626)
at hudson.plugins.ircbot.PluginImpl.<init>(PluginImpl.java:18)
at java.lang.J9VMInternals.newInstanceImpl(Native Method)
at java.lang.Class.newInstance(Class.java:1549)
at hudson.ClassicPluginStrategy.load(ClassicPluginStrategy.java:416)
... 9 more}}

Downgrading to 1.30 worked.

I know that IBM J9 might be not supported, but I think that instant-messaging should support 1.6 version of Java.