In reply to ochils ben:
We rode past Shenavall in May as part of a Ullapool - Kinlochewe 75 mile epic. If you take the double track from Corrie Hallie, as Nick B has said, it is rideable the whole way (and the section up the valley to Shenavall is really nice).
There are indeed some big signs on the Poolewe side telling you not to ride on anything other than the estate roads. They certainly aren't worded as "advisory" and so seemed to me to be entirely against the access rights legislation.
In any event, I wouldn't ride in from that way. It was a fun way out, particularly the descent to Fionn Loch, but too much hike a bike otherwise.