Actually, for hair or similar very thin object it is an antialiasing problem, Sometimes Gi has problems with those objects too but is mostly antialiasing.
Now if you are doing animation, I would not recommend using Irradiance for hair object, I would recommend brute force.
Could you post some images and setting of your scene?
Did you try checking the box of detail enhancement in irradiance options? That will brute force those small areas for better light distribution and antialiasing.