A website I'm not familiar with known as The Devils Eyes says McNairy is actually playing Jimmy Olsen, Clark Kent's pal at The Daily Planet. But this wouldn't be the Olsen we're familiar with, as this one would have had his legs injured in Superman's fight against Zod and now walks around using prosthetic limbs. It makes sense, and would serve as a constant reminder to Superman of the ramifications of his powers if left unchecked. The character has been around since the 1940 Superman radio show, and has been portrayed on big and small screens multiple times. Sam Huntington played Jimmy Olsen in Superman Returns, and Shawn Ashmore played him on Smallville.
As much as I would like to see McNairy as Flash, this idea makes more sense and is cool in its own right. Just file this one away until somebody actually confirms something about this movie. Batman v Superman: Dawn of Justice opens March 25th 2016.