Simply navigate via console to your app’s directory, then enter:

bundle exec irb -I. -r app.rb where app.rb is the entrypoint to your Sinatra.rb App.

On Heroku things do not get complicated, too. Make sure heroku is configured and you are in the directory of your app. Run heroku run bash and wait for the bash to load - you should enter in your app’s root directory. Confirm this by typing pwd or ls -lha, else navigate to the entrypoint of your Ruby App.

bundle exec irb -I. -r app.rb where app.rb is the entrypoint to your Sinatra.rb App and you are good to hack away.