The Active Admin
gem creates default UI for data administrators:
Active Admin is a Ruby on Rails plugin for generating administration style interfaces. It abstracts common business application patterns to make it simple for developers to implement beautiful and elegant interfaces with very little effort.
It is not like Joomla admin pages, which allows the admin to change/add the pages users can see, unless their are data-driven in an application specifically written to enable that.
You can look for libraries which solve this problem, or integrate rails with a CMS application (like this: http://joomlarails.com/)