Melinda in 1860 census Alexander McCann Stewart (son of Jane McCann and Robert Stewart of Killinchy) can be found living with Samuel Murdock who owned a ranch in Colusa County, California. The Census lists Samuel Murdock age 43, his wife Anne S.,age 29, Eleanor age 19, and William, age 1. All born in Ireland except the baby.
In 1869 Jane McCann Stewart wrote to one of her daughters that " Sam says we need not be suprised if we hear of Alexanders marriage" which sounds as if "Sam" kept in contact with Robert Stuart and his wife.
My assumption at this point is that maybe your Ann Stewart and our Robert Stewart both from Killinchy were related!
Other names commected to this family from Killinchy include McKee, Potter, McCann, Lowry, Jamison and many others.Immigration path from what we know is port of New Orleans, Indiana, Illinois, Missouri.
If interested in exchanging info please drop me a line. Mickey McKee