System.Environment.Is64BitOperatingSystem was added in .NET 4.0: https://msdn.microsoft.com/en-us/library/system.environment.is64bitoperatingsystem(VS.100).aspx
Okay, that idea isn't going to work (updating IrrKlang). I can't figure out a way to include both the 32-bit and 64-bit DLLs in the same project and get VS to automatically reference the right one when compiling. In a C/C++ project this is easy, just change the library path, but the .NET languages use the braindead References system instead. The only workaround I can think of is to create separate project files for the 64-bit build, but with all the plugins that would be a mess.