According to William A. Dempsey death certificate, parents are William Dempsey and Matelda Patrick. According to his brother's (Thomas N. Dempsey) death certificate, parents are Patrick Dempsey and unknown. According to brother's (Patrick Lee Dempsey) death certificate, parents are Patrick Dempsey and Lizzie Green.
Couldnt find death certificate for brother John Dempsey.
Look at Dempsey families in Cherokee County, AL in 1880 census.

I'm not related. just trying to help.
John C. Awbrey