I am also researching the Shirers in Orangeburg. My information shows that John Adam Shirer married to Mary Ann Jackson is the brother of my third great grandfather Aaron James Shirer married to Mary Zeagler 1841. I have found an elderly woman listed on the 1870 census with John Adam Shirer named Allice aged 70 born around 1800. I am still researching who her husband was.
Jacob Shirer is listed as having been granted property in Orangeburg around 1790 or so.
Hope this helps a little.
Get in touch with me if you want more help.
Betty Fox (Oregon)