There are so many Agnes' out there that it's hard to determine whose who. Halbert, to my understanding (no proof), was married to Agnes Gilespie. This is not my direct ancestor so I don't know much except what I read from others. So, much of this information is from the "collectors of information" and it's doubtful that most of it has been researched.
I've also been told that Halbert McClure was a brother to my ancestor, James McClure, who was also married to an Agnes. We've found no maiden name for this one nor have we found that there is a connection between Halbert and James. You will find the name, Agnes, mentioned a lot in the Chalkley Chronicles of Virginia.
Good luck with your research.