I can confirm that, more or less. The set being built at Magheramorne will be our Castle Black. Not actually THAT massive a castle -- much of Castle Black will be created in post with CGI -- but it will come complete with a fully operable winch elevator, David and Dan tell me.
Google tells me that "the gobbinsite at Magheramorne quarry is found as a rare filling of vugs and vesicles." I have no idea what that means, but hey, it sounds like it came straight out of someone's fantasy novel. Gobbins, vugs, and vesicles all in one place. I hope they don't attack the castle. Those gobbins are mean sonsobitches.
Let me caution anyone reading this, however -- please DO NOT go rushing off to visit Magheramorne. Quarries can be dangerous places, and construction sites doubly so. We don't want any fans getting hurt. Besides, why spoil the illusion? Castle Black will be best viewed on screen, in all its glory.
(And no, HBO will not actually be building a 700 foot high wall of ice. We tried, but the cubes kept melting).