The Kerrygold butter is not 100% grass fed all year round because during the winter months the cows need either grain or some hay to eat to keep them thriving.
So the cows are not 100% grass fed all year round to produce the Kerrygold butter.
Still Kerrygold butter which is mostly grass fed half the year and fed on hay as well as probably some grain is still healthier butter than other butter.
I find that Kerrygold butter tastes better than other butters as well and is what I mostly use because it is the healthier option for butter because of it being mostly grass fed.
Kerrygold butter company did admit that the cows that the butter comes from are not 100% grass fed even though they used to claim that.