If you do use hemlock, use it right away. When it’s “green,” it’s soft and easy to cut (although juicy), and easy to nail or put screws in. Once it’s dry, it’s like rock, and it does splinter.
We used it for stall walls years ago, and it certainly was tough. Didn’t get chewed anything like as much as the pine.
You could do the boards in hemlock and then do a top rail in ironwood.