irb.sh

1.8.7 :001 > autoload :MyLibrary, 'my_library'
 => nil 
1.8.7 :002 > module FooModule
1.8.7 :003?>   def foo
1.8.7 :004?>     puts 'new foo'
1.8.7 :005?>     end
1.8.7 :006?>   end
 => nil 
1.8.7 :007 > class MyLibrary
1.8.7 :008?>   include FooModule
1.8.7 :009?>   end
 => MyLibrary 
1.8.7 :010 > MyLibrary.new.foo
original foo

my_library.rb

class MyLibrary
  def foo
    puts 'original foo'
  end
end