application_controller.rb

# Filters added to this controller apply to all controllers in the application.
# Likewise, all the methods added will be available for all controllers.

class ApplicationController < ActionController::Base
  helper :all # include all helpers, all the time
  protect_from_forgery # See ActionController::RequestForgeryProtection for details
  
  before_filter :base_auth
  
  protected
  
  def base_auth
    warden.authenticate! :scope => :base
  end
end

devise.rb

require 'base64'

class FakeModel
  def id
    'fake_id'
  end
  
  def self.find(*args)
    nil
  end
end

Warden::Strategies.add(:required_basic_auth) do
  def authenticate!
    if scope != :base
      return
    end
    
    auth = request.headers["Authorization"]
    
    if auth and auth =~ /^Basic (.+)$/
      credentials = Base64::decode64($1)
      login, password = credentials.split(":", 2)
      
      if login == 'foo' and password == 'bar' # change this for a login/password that you want
        success!(FakeModel.new)
        return
      end
    end
    
    custom!([401, custom_headers, ["HTTP Basic: Access denied.\n"]])
  end
  
  protected
  
  def custom_headers
    {
      "Content-Type" => "text/html",
      "WWW-Authenticate" => %(Basic realm="Application")
    }
  end
end

Devise.setup do |config|
  # ... other stuff on your devise configuration

  # configure warden
  config.warden do |manager|
    manager.default_strategies.unshift :required_basic_auth
  end

  # ... maybe here has more config... just ensure the above line
end