There just happened to be "an old bloke building a stone wall" who happened to know it is called "Fiann's Arrow". It's the glen farmers who repair the dykes and there's been no wall building there that I know of, so I'd be inclined to take Mr Clyne's story telling with a pinch of salt.
There's lots of similar stones like these dotted around the area on opposite sides of the hill. They're the result of glacial transport and deposition. You can see a similar one here by the side of the road on the A827 at grd reference NN 64227 37630
Sandy
Something is not right. This message is just to keep things from messing up down the road