Use jrubyc to compile your main ruby script to class files. That should do.
I agree but there's no way to really recover the original ruby script. At best you'll retrieve what you listed and the string constants.
It's the easiest form of obfuscation I know for jruby.
If you have another suggestion, I will test it
I almost forgot... Have a look at jd-gui if you're using Windows. You'll get a much better java decompilation than what you suggested.
I'm not the creator of this thread, but I believe settings should be part of a configuration file instead of being in the code itself.
Database.yaml in rails is a typical example of such approach.