[Zardoz]

Originally Posted by Commodore

My bet is that Earth ships became equipped with deflector shields sometime after the Coalition of Planets was formed. Who knows, perhaps the advent of deflector shield technology aboard Earth ships was the turning point in the Romulan Wars...

I would assume that the technology would have come from, or been adapted for use by the Vulcans who had a vested interst in the outcome of that war.

ENT made it clear the sheilds were know of by humans, but the technology did not exist in any form on human ships.