Hi!
First of all I was quite surprised to read the history of the Shirer family because there is a really good chance we are related.
John Shirer, or Johan(nes) Schurer what his real name probably should be, went to the USA like many more Schurers in that time. If he's from the Dutch or German brance of the family I can't tell (yet). I'll keep in mind 1735. If you ever find out his exact date of birth please let me know.
The funny thing is (especially reading the HOWTO on pronounciation) that Shirer comes really close to the Dutch/German way of pronouncing Schurer and it's quite understandable that on Ellis Island names where less important but getting in more so that's probably why your surname is Shirer now.
You also have to know that John is the english word for Johan or Johannes. I use it too and it's a really common firstname for Schurers throughout the centuries (dating back to +-1470)
Schürer, Schierer, Schorer, Schuerer, Scheurer are all writing variants on the same common surname: Schurer
As for the meaning of the name stated by Lois June Powell Orton in 1979: "one who had charge of the barn or granary" could be true. The name Schurer dates back from before 1400. The Dutch word 'schuur' means barn and 'schuur-er' is pronounced exactly as Schurer.
If you want to learn more about Schurer please drop me a line.