It might be helpful to have either your husband, or father-in-law take a DNA test. There are a lot of Stewarts coming from Ireland, and settling in different parts of the US. I can give you a starting point because I have a John Stewart in my family tree. I also have a Charles Elmer Stewart in my tree, who was my grandfather....John Stewart B: 1743 in Procoles, Atrim County, Ireland
D: 1818 in Ireland...Hope this is of some help, but do suggest a DNA test in order to find the right branch to follow.
Let me know, and keep in touch.
Phyllis (Stewart) Lester