Nant Tarw can be translated as "bull brook", and Corn Ddu is known as "black horn". Welsh translations are notoriously iffy, so there may be a connection, or not. The dates of sunrise through the beacons as seen from the stone row might be a better fit.

It's fascinating. As you've said on many occasions, I think the watercourses/sources are the key to these sites. The mountains seem almost incidental at Nant Tarw, although when I was there they were obscured by clouds so I probably didn't get a proper feel for their relationship to the site. I would love to go back there, it's a damn long walk though.