I recently started using JRuby for a Rails project and overall the experience has been excellent. Using RVM, you can just switch to jruby to build a new project (rvm use jruby), and just about everything will work the same. One of the big features of JRuby is that you can bundle your entire app, including JRuby itself, in to a WAR file that Java servers like Tomcat and Glassfish can serve up, so your app can be distributed on to servers that only have Java.
After you have JRuby installed, simply install the gem warbler. You'll then get a command line tool, warble, to generate war files from your project. Simply run warble from your project's directory, and a war file will be produced. It's as easy as that!
Another great feature is that the Ruby code can be compiled down to Java class files, so your source code is not visible. This is great for distributing on to a server where other companies will have access that you don't want seeing your source code. However, this is not working for me. Warbler should support this, just run "warble compiled war" from the command line in your projects directory instead of "warble". This will produce a war file with both .rb and .class files. The .rb files however are simply stubs that require the .class file, none of your code is in there. But, for me, it's not generating .class files on all of my controllers. I've entered an issue for this at https://github.com/jruby/warbler/issues/72.
I will be making another post on how to do database migrations on the server you're deploying to.