It's possible to take a car to Barra, Harris, Lewis, North Uist and South Uist from the mainland by ferry. The ferries are operated by Caledonian MacBrayne, who also offers detailed information about getting to and from the ports by car.
There are several car hire operators on Lewis, Benbecula and the Isle of Barra if you wish to travel to the isles as a foot passenger but decide to rent a car for few days, and most companies provides an airport pick up service too.
Driving around the islands is a great way to explore off the beaten track and enjoy the wild Outer Hebridean scenery. The roads on the islands vary between smooth, standard double tracks on the main routes to narrow, single tracks in more rural areas. Make sure that you read about driving in Scotland, and are aware of how to use passing places on single track roads and watch out for livestock on the roads in rural areas.
Travelling between the islands by car is becoming easier and easier with bridges, including a crossing to Scalpay and causeways to Berneray and Eriskay, ferry terminals at Stornoway and Leverburgh, and car ferries on the routes between Harris and the Uists and from the mainland to Barra and Stornoway.
The Traffic Scotland site is a great source of up-to-date traffic information.