Jason, The narrow gauge line will certainly be the best way to approach this extraordinary old course. The trouble is that the railway ends at Blenau Ffestiniog, not Ffestiniog itself, so you are in for a two or three mile hike across pretty mountainous roads to reach the golf course. However, it was dead flat at Harlech and not too strenuous at Porthmadog (though there is wonderful use of the dunes and there are some great views), so you've got plenty of energy still in the legs.
I don't know whether it is still true, but when I was last there, some ten years ago, there had never been a hole in one on the course.