How do I come up with them so fast? It's easy -- I just slack off on the work I should actually be doing. (I hope my boss doesn't see this.)
Anyway, Brandy Hall is the correct answer. One thing I learned while putting this riddle together is that "Gorhendad," as in Gorhendad Oldbuck -- the Hobbit who first started delving Brandy Hall -- translates as "great-grandfather."
The last line, by the way, refers to Merry's expansion of the library after he became Master of the Hall.
SL