My ~/.irbrc with the Rails app name and environment. A useful bit of paranoia to help prevent operator errors.

irbrc-rails-env.rb

if defined?(Rails) && Rails.env

  reset = "\e[0m"
  color = case Rails.env
  when 'development', 'test'
    "\e[36m" # cyan
  else
    "\e[31m" # magenta
  end

  app_name_env  = "#{Rails.application.class.parent_name.underscore}"
  app_name_env << "#{reset}:#{color}#{Rails.env}#{reset}"

  IRB.conf[:PROMPT][:RAILS_ENV] = {
    :PROMPT_I => "%N(#{app_name_env}):%03n:%i> ",
    :PROMPT_N => "%N(#{app_name_env}):%03n:%i> ",
    :PROMPT_S => "%N(#{app_name_env}):%03n:%i%l ",
    :PROMPT_C => "%N(#{app_name_env}):%03n:%i* ",
    :RETURN => "=> %s\n"
  }
  IRB.conf[:PROMPT_MODE] = :RAILS_ENV
end