A relatively easy walk S-SW from Glenmacnass Waterfall carpark along a relatively well-defined path, until I left the beaten track just before the 668m hill, heading N-NW to make it directly to the lake shore. The ground turns boggy in places, you can be waist high in bog before you know it, and with the 'normal' hidden bogholes you get in this terrain, extra care is needed. There were no paths that I could see to help. Once you get to the lake, also be careful that in places the stony-looking shoreline is not like a beach - it's in fact boggy mud. I found that out the hard way :-P
Although surrounded in mist, the silence of the place was fantastic, maybe enhanced by it even. Through occasional parting of the clouds, I could see where the lake drained away to Glenmacnass river far across the lake. The cache is in a container which I think you will agree respects the tranquility of this place 
The usual logbook and a few goodies are there also. Note that I've posted two pictures of the approximate location, as scrambling along the shore of the lake isn't as much fun as it sounds 