consolize.rb

require 'rubygems'
require 'irb'

def consolize &block
  
  yield

  IRB.setup(nil)
  irb = IRB::Irb.new
  IRB.conf[:MAIN_CONTEXT] = irb.context
  irb.context.evaluate("require 'irb/completion'", 0)

  trap("SIGINT") do
    irb.signal_handle
  end
  catch(:IRB_EXIT) do
    irb.eval_input
  end
end

consolize do
  # require whatever you need
  require 'mongo_mapper'
end

# run from command line
$ ruby consolize.rb