Thanks for the lookup, Shirley. However, I think the fact that William's wife Mary was born in KY in the late 1780's or early 1790's convinces me that I was barking up the wrong tree.
I had learned from another source of the marriage in Madison County (date unknown) of William Kirkley and Mary Cowan. I assume that these are the same William and Mary Kirkley that are mentioned in these two censuses. My Cowan ancestors had lived in Madison County during the 1840's and 1850's, and a sister of my gg-grandfather was married to a man whose first wife was Rachael Kirkley. This got me to thinking that William might be related to Rachael and Mary might be related to my Cowans. But my Cowan family didn't immigrate until 1801, and lived in NY until the 1830's, and to my knowledge none of them ever lived in KY--certainly not born there before 1800. So it's probably just a coincidence of names.