There is a large stream to the east of the carvings, in the valley floor, and there's nothing marked on the survey maps further east of this watercourse.

There's also a spectacular mountain pass to the east with a lake in front of it. This is taken looking south east but you can see the valley floor and part of the pass to the east. Oddly enough, this is the only panel I found that doesn't face the same direction as the rest, though it may not be earthfast. http://www.themodernantiquarian.com/post/57228/images/derrynablaha_f.html