If you are walking through Birnam Glen, it's well worth a little diversion to find this delightful waterfall. From the bridge that crosses the burn before entering the Ladywell Plantation, you can simply follow the burn for a few hundred yards until you reach the falls - but be aware there is no path and the walking is rough, with steep scramble at the end.
UPDATE! The easiest way to reach the falls is just to carry on on the path towards Balhomish (ie left at the fork with the sign post) and after a few hundred yards you will come to a bend in the path, with a deer fence heading downhill to the right. There's a signpost thoughtfully put up by we don't know who, but even if it's disappeared, just follow the fence downhill then round to the left and very soon you'll be at the top of the falls.
This lovely video was done by Phil Brammer, and you can read about his adventure finding the falls by clicking here.