There are about zillion William Stewart's as I have found researching my ggrandfather who's family was from Scotland and emigrated to Ireland. Where did your Stewart's emigrate from? Do you have any other information that you can share. There are several Sir William Stewart's found on Wikipedia which may give you some leads.
You may also want to try one of these sites: http://churchrecords.irishgenealogy.ie/churchrecords/http://www.igp-web.com/IGPArchives/