While Ruby on Rails is primarily a web app development framework, it is possible for Ruby on Rails developers to build mobile applications using technologies such as RubyMotion or React Native.
However, it's important to note that building mobile applications is a different skill set from building web applications, and developers who specialize in mobile application development may be better suited for this type of project. That being said, Ruby on Rails developers who have experience with mobile development frameworks like RubyMotion or React Native can certainly contribute to mobile application development projects.