For anyone going to ByN in the next week or two (including those going to the Working Meet), the road up to the Lake from Trefriw is still closed, and under repair. The repair is expected to take about three weeks.
So, as previously posted, the best way of getting to the hut is by using the forestry road. To do this, get to Geirionydd, either from Trefriw or from the road near Gwydir castle. The latter route may be a bit more straightforward. Then drive up the left side of the lake, crossing over at the end to get onto the forestry road, which winds its way to exit on the Crafnant road opposite the car park. You will need a map to get to Geirionydd, preferably an OS map!