Target is Netbeans with JRuby, not JRuby from terminal or command line.
The promise of JRuby/Netbeans I think is that we should be able to write JRuby code in Netbeans, press a button and have
a jar ready to deploy. Following discussion on mail lists it may be possible use F11/Build Button for this purpose. 
http://www.infoq.com/articles/jruby-deployment-with-webstart includes discussion which may be applicable.

Quote:
One of the exciting new features of JRuby 1.1 is the ahead of time (AOT) compilation support. Currently, just in time
compilation in JRuby is restricted to 2048 methods, ahead of time compiling can help to mitigate that restriction.
jrubyc, the JRuby compiler, is still under development, so I'd advise to use the latest JRuby release available.
Compiling a plain Ruby file to a classfile is as simple as invoking the compiler with the script(s) as arguments:

jrubyc rob.rb 

This will create a ruby directory containing a rob.class file. Instead of including the ruby directory into the
jruby.jar as we did above, we're going to create a separate  Jar to hold the application. After all, modifying the
existing Jar doesn't really seem to be such a neat solution. Jars can be made by the tool of the same name:

jar -cfe rob.jar ruby/rob.class ruby 

This will create a small Jar called rob.jar with our class inside, adding the ruby/rob.class as the main-class to be
specified in the Manifest. This allows us to simplify the invocation, as we can now simply point to the class and don't
have to speficy the require on the command line anymore. To execute it, we have to make sure that rob.jar is on the
classpath:

java -Djruby.objectspace.enabled=true -cp rob.jar:jruby.jar ruby.rob