We are using PJIRC v2.2.1 which was quite popular back in time, that's an applet and the Java plugin is required, please visit www.java.com for more information.
This is an old GNU applet and the certificate isn't valid anymore, althought it's still working but don't be surprised to see the usual warning window :

Once you accept to run/execute the applet (as if that would not be enough), another window pops up asking if you wanna block some 'potentially' unsafe components.

You shouldn't block anything and the PJIRC applet can be safely ran, I don't know why all those useless warnings, I think that some notification windows will always popup the first time the applet is run, seems unavoidable as user confirmation is required for the thing to run.
Sometimes it can be difficult to connect at the Undernet Network, what you wanna see is nothing else than the magic brown lines as shown below.

Any other lines usually means that the connection isn't going well, you can try to reconnect by choosing another server. At last solution you can also reload the page by using the button, most of the time the connection is okay but sometimes one have to use some patience, althought it will connect

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Looking forward to chat with you.
maz