Wanna help improve JRuby? Make sure you read

As you can see, you need to create a new “Remote Java Application” configuration, setting the port to the same one you used to start the debugger session - 4000 in our case. Don’t forget to add JRuby’s source code in the Source tab on this same screen.

Once you’re done, set a breakpoint anywhere in JRuby’s sources and run the newly created configuration.

That’s it! Enjoy!