I have observed this behavior on irc.gimp.net as well.

I think we have a similar issue, internally reported as: https://www.drupal.org/node/2159903

When I restart jenkins I get this RuntimeException:

IRC notifier plugin: Sending notification to: #aegir
IRC notifier plugin: There was an error sending notification to: #aegir
java.lang.RuntimeException: Waiting for mode response interrupted
at org.pircbotx.Channel.getMode(Channel.java:133)
at hudson.plugins.ircbot.v2.IRCConnection.send(IRCConnection.java:276)
at hudson.plugins.ircbot.v2.IRCConnection.send(IRCConnection.java:268)
at hudson.plugins.im.IMPublisher.sendNotification(IMPublisher.java:379)
at hudson.plugins.im.IMPublisher.notifyChatsOnBuildEnd(IMPublisher.java:585)
at hudson.plugins.im.IMPublisher.notifyOnBuildEnd(IMPublisher.java:304)
at hudson.plugins.im.IMPublisher.perform(IMPublisher.java:291)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:45)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:804)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:776)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:725)
at hudson.model.Run.execute(Run.java:1709)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:231)
Caused by: java.lang.InterruptedException
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:979)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1281)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:207)
at org.pircbotx.Channel.getMode(Channel.java:127)
... 15 more

An older RuntimeException from the jenkins log, I guess the first notice of when the connection was lost.

Aug 6, 2014 2:31:18 AM hudson.plugins.ircbot.v2.IRCConnection send
WARNING: Bot is configured to use colors, but channel #aegir disallows colors!
Exception in thread "bot1-input" java.lang.RuntimeException: Waiting for mode response interrupted
at org.pircbotx.Channel.getMode(Channel.java:133)
at org.pircbotx.Channel.getModeArgument(Channel.java:182)
at org.pircbotx.Channel.getChannelKey(Channel.java:239)
at org.pircbotx.PircBotX.shutdown(PircBotX.java:2872)
at org.pircbotx.PircBotX.shutdown(PircBotX.java:2833)
at org.pircbotx.InputThread.run(InputThread.java:115)
Caused by: java.lang.InterruptedException
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1279)
at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:207)
at org.pircbotx.Channel.getMode(Channel.java:127)
... 5 more

Got feedback in another issue: https://issues.jenkins-ci.org/browse/JENKINS-26491

The build-timeout-plugin is not a solution here.

I therefore would like to request some sort of timeout in the instant-messaging plugin.

Herman, are you sure you are talking about the same issue?
Why would you even think that the build-timeout plugin could be a solution to the IRC plugin disconnecting after a long idle time?

Kutzi, I agree that it's not a proper solution. Some form of re-connect would probably be needed.

What I had hoped the build-timeout plugin to solve is to stop a build that is hanging on the notification. A failed build would alert me of an issue, letting me restart Jenkins if needed to get the IRC connection back. The ironic thing is that the notification is an after-build step, that's why the build-timeout plugin did not help.

Without a timeout it could be a week before anyone noticed that the builds have stalled.