Marilla is most likely an elaboration of the name Mary. See Also: Mary
Marilla
If I had another daughter, I would name her Marilla Anne, from Anne of Green Gables. Per the books, it can also be shortened to Rilla as a nickname. I think it's a beautiful, classic choice that is unusual enough to not be associated too strongly with an age group.
Marilla is most likely an elaboration of the name Mary.
See Also: Mary