Check out this railscast if you're competely new to JRuby.
I haven't used JRuby myself but I know people who have used it in production and it does seem to work well. A couple of things
I've heard because of the way the JVM works the startup time is quite slow as it optimizes things. And, you might run into compatibility issues with certain gems.
Can we not use Rails if we do JRuby?
You can use Rails in it's entirety, you just also have access to all of the Java libraries should you need them from Ruby.
I'd seriously question why you need to access Java from Ruby though. Obviously, including Java in your stack is adding complexity you might be better off avoiding. You say Ruby has limitations, and that may be true but what in particular are you referring to?
Working with services / api's might be a better option, you'll need to do some research.