An amazing day. After Lairg you're into near-true wilderness; the A-road becomes single-track with passing places, and there is very little traffic and many spectacular views over broad, flat... tundra? Almost-tundra.
The climb - to approximately the currently (possibly?) closed Crask Inn - is imperceptible, and then you have some incredible descents.
Accommodation in the wonderful Tongue SYHA-affiliated hostel, which sells frozen home-made meals.
(Take a stroll west along the causeway for great views of Ben Loyal.)