Thanks for replying.
John Herrin's service record is for John Daniel Herrin, and only says died at Johnson Island, Pointdexter's unit.
This is actually his brother, Daniel, who apparently lied about his name because he appears to have previously taken the oath.
I often visit Washington DC and try to visit the archives on each trip. I have slowing been working my way through the confederate service records of all John Herrin/Herron/Herring. Slow process.
A distant relative says he believes John was in the Union army. More records to look through.