A tutorial on JRuby with Nick Sieger holding your hand is just great. This guy knows JRuby inside-out and he has an answer to even the most difficult of questions.
After a short 15 minute introduction, Nick left us with our Rails app to start it with JRuby on a mongrel. I’ve been toying around with JRuby a bit before, and this was rather easy. Some people, however, encounter some issues with their apps because of incompatible gems, like OpenSSL or RMagic which are not available for the Java platform.