In reply to Flamingsarah:
If you're off to Barra from South Uist, especially with a car it's worth booking the ferry a bit in advance. There are 2 crossings, but they do get booked up quick in the summer months.
Probably get on as foot passenger at short notice, but then it's getting to the crags on the island. Can recommend booking a days sea kayaking on Barra, great way to see the island and wildlife, again worth booking in advance.
Have a great time (hopefully) a beautiful place.