Have you checked the other forums - like FITZGERALD, or GERALD or JARRELLS?
On the JARRELL forum, I found postings by Wiley Aston Jarrell which mention both Zacharia and Zachary and Zacheriah - Zachariah -- there are many variant spellings for the given name, too -- not just for the surname.
I encourage you to explore the dozens & dozens & dozens of postings that Wiley Aston Jarrell has made -- evidentally an attempt to identify ALL Fitzgeralds - Jarrells, Garratts, ETC.
I think that most of his postings are on the JARRELL family genforum -- and I think I found a few FITZGERALDS in his posts where the records identified them as Geralds & Jarrells.
Just be sure that you find out ALL the variant spellings for Zacheriah -- because evidentally there are plenty.
I did a search for Zacheriah and for Zacharia -- and there are about half a dozen posts at the JARRELL family forum for just those 2 spellings.