My first thought when I saw this picture was "Hey...I didn't know Captain America fought in the Civil War too!"
A Northern state militiaman, unidentified, possibly a fireman. Sadly there's no indication in the picture of what unit he belonged to, whether this shirt was worn by other men of his unit as well, or if he was so patriotic he just had to put a shield on his chest. We will never know what some of these men were thinking.
*Addendum: This man may have been in a company of Fire Zouaves who put the symbol of their fire house on the bibs or "plastrons" of their shirts...one member of the Authentic Farbs Facebook group thinks it might have been the 73rd New York Volunteer Infantry, formed from a fire company calling themselves "old Americus" firemen.
This image, found on Pinterest, points to an eBay auction. You can own this odd piece of history for $299. (Link: http://pinterest.com/pin/513762269964661865/ )