Adding class/module to lib folder of rails application. By adding the lib folder to autoload_path, you don't need to require the file (require 'file') in other classes.

application.rb

require File.expand_path('../boot', __FILE__)

require 'rails/all'

Bundler.require(:default, Rails.env)

module TestApplication
  class Application < Rails::Application
    #add the lib folder to autoload path
    config.autoload_paths += %W(#{config.root}/lib/assets/ #{config.root}/lib/)
  end
end

hello.rb

# lib/hello.rb
module Hello
  def say_hello
    puts "Hellooo...."
  end
end

irb

####  Access class placed in lib folder
2.0.0-p247 :001 > test = Test.new
 => #<Test:0xb490204 @x=12> 
2.0.0-p247 :002 > test.x
 => 12 

####  Access module placed in lib folder

2.0.0-p247 :004 > include Hello
 => Object 
2.0.0-p247 :005 > say_hello
Hellooo....

test.rb

# lib/test.rb
class Test
  def initialize
    @x = 12
  end
  def x
    @x
  end
end