In reply to Garston:
Could be anything on Craig Y wrysgan or, if you walk off from the top of the routes, Clogwyn yr oen. The tunnel is part of the massive incline linking the slate quarry South of Cwmorthin with the railway at Tan Y grisiau. It's marked on the OS 1:25k map at SH 678454
The most popular routes using the tunnel descent would be White streak/Honeysuckle corner or Y Gelynen.
Post edited at 22:06