No special access issues
This is not one for the typical gym rat and an approach that should be treated with some caution. There are two options:
🚶♂️1/ From the campsite. Cross the bridge and turn left, passing several gates following the path until you find yourself on top of Clogwyn y Fulfran. Where the path trends rightwards, follow the (very) steep vague path down and left, crossing felled trees and being especially cautious not to topple over head first down the hill. The cave is to your left at the lakeside.
🚶♂️2/ From the southern end of the lake. Park in the layby and follow the path into the field. Cross a small bridge and turn right, following the lakeside across a boulder field to arrive at the cave.