V1 Refrence
 

I'm sure there is a perfectly reasonable explanation, but I haven't found one...

Why does V1 use IAS rather than groundspeed. If it is to do with stopping distance, then surely the airflow over the plane has little effect on it. If you had a headwind, then you would reach V1 at a slower groundspeed than with no headwind, so you would have 'more runway' ahead of you stop in.

Or is it simply a case of groundspeed is IRS derived, IAS is from a pitot tube?

V1 is an airspeed because it's airspeeds which are presented to the pilot, and airspeeds that he flies the plane to.

Headwind/tailwind is factored into calculated stop and go distances, so your V1 may change to rebalance the stop and go performance accordingly.

Another reason for using an airspeed not a groundspeed is that some parameters which determine V1 (such as Vmcg or Vmu) are airspeed driven, not groundspeed, and so a "groundspeed V1" would need to account for airspeeds anyway.