it doesn't have to be complicated

.irbrc

# Append the user's .railsrc.rb to the load modules, that way it's loaded
# after the rails ENV and everything else...
if $0 == 'irb' && ENV['RAILS_ENV']
  IRB.conf[:LOAD_MODULES] << File.dirname(__FILE__) + '/.railsrc' # ~/.railsrc.rb, requires .rb
end

.railsrc.rb

module UserPasswordResetter
  def new_password_from_irb=(new_password)
    self.password = self.password_confirmation = new_password
  end
end

User.send(:include, UserPasswordResetter) unless User.included_modules.include?(UserPasswordResetter)