"Try searching Google Earth for Ballygrand, Ballygran, Ballygrat - you won't find any of them. These are made-up names."

I just did. There's a Ballygran estate, not far from Kilkenny.
I don't think it's of much relevance, however. The insistence that it's Ballygrant on Islay is kind of putting me off even considering it.
Anything is possible, but I think the point that "Ballygrand" didn't enter into the equation until the Clancys' version is quite valid.