A good rule of thumb for low levels is 1-6-6, ie:
True Air Speed is 1% higher than IAS for every:
6 deg C over ISA atmosphere and,
6 Flight levels over FL00.
You can also use the whizz wheel.
For low-level bimbles in Ireland, I suspect that errors in the forecast wind will swamp such subtleties.
I would worry more about understanding density altitude!