Running a JRuby application is similar to running any Ruby application. The main difference is that you set the Application Server to Trinidad.
To run a new JRuby application on Engine Yard Cloud
In your Dashboard, create an application.
Create the environment, setting the Application Server to Trinidad. When the Application Server is set to Trinidad, the Runtime defaults to JRuby 1.6.5 compatible with Ruby 1.8.7. For JRuby compatible with Ruby 1.9.2, set the Runtime to JRuby 1.6.5 (ruby–1.9.2-p136).