Here's a quick fix for glass normal mapping

BlackhartFilms writes:

Normal Maps with the Glass or Refraction Shaders in Cycles are broken; this is a long standing known bug in Blender. Bump mapping works fine, but the Normal Map node breaks these shaders, making them opaque and blackened. This video demonstrates the issue, discusses potential causes, and shows a really quick and easy fix using the node editor to allow you to still gain all the benefits of normal maps while using the Glass Shader, if you wanted to create effects such as water, frosted glass, stained glass, etc...

Update: It appears that this issue may have been fixed for the 2.78RC; this solution is for 2.77a and prior. If the issue has been resolved for 2.78, wonderful, this solution will be no longer necessary. If the issue persists on your specific setup then this is the quick solution to fix your renders.