No special access issues
Access is generally from the forest track running from the Siabod Cafe along to the bridge at the bottom of Plas y Brenin. Parking can be found at either end or indeed in the woods proper (if your car can handle the rough track). Cairns are located around the woods to direct you from the path.
The Blair Witch boulder, Brenin boulder, Bryn Engan boulder and Mymbyr boulders can all be accessed from the lower forest track.
The Prowess Area is accessed from the rising track coming from the Siabod Cafe. Go through the gate as the path rises and a cairn will appear up and left.