Is Adobe illustrator GPU acceleration available on Mac mini with iris graphics card
The Mac Mini supports OpenGL 4.1, which is required, but may not have to enough Video RAM (VRAM). Low End Mac and Every Mac differ on how much VRAM the Mac Mini has.


If Every Mac is correct, GPU acceleration is supported.
If Low End Mac is correct, GPU acceleration may be disabled.


OpenGL 4.0+ Required - OK

See the Mac OS section of Illustrator Help  / 
System requirements | Illustrator:


  Optional: To use GPU Performance: Your Mac should have a minimum of 512 MB of VRAM (2 GB recommended), and your computer must support OpenGL version 4.0 or greater.
  
  To check for VRAM values:
  
  
  Mac 10.9: Choose Mac > About This Mac > More Info (Graphics information).
  Mac 10.10, 10.11: Choose Mac > About This Mac (Graphics information).
  
  
  To find out if your computer supports the required OpenGL version (4.0 or later), see this Apple Support document.


The linked Apple Support document states the Mac Mini (Late 2014) with Intel Iris Graphics supports OpenGL 4.1. So the GPU acceleration is technically possible.

512MB VRAM Required - Maybe?

Low End End claims 288 MB is available to the graphics card; this is lower than Adobe's recommended 512 MB:


  
  GPU: Intel HD 5000 Graphics/Intel Iris Graphics
  VRAM: 288 MB DDR3 SDRAM (shared with main memory)
  


However, EveryMac report this Mac Mini has access to 1.5 GB of VRAM, shared from the Mac's RAM:


  This model has an Intel Iris 5100 graphics processor that shares memory with the system.
  
  Apple reports that this model uses "Intel Iris Graphics" and allocates up to 1.5 GB of system memory for video function.