Wild red deer in Ireland and Scotland are considerably smaller-bodied than wild red deer in Europe--or on game farms in the U.S. They're pretty much the size of mule deer. A .260 is plenty, just like it is for mule deer.
Do you mean they're considerably smaller-bodied than wild red deer in mainland
I shot an Iberian Red Deer in Spain and it was mule deer sized. I figured there was quite a difference between red deer on the Iberian Peninsula and in say, Germany or the Czech Republic, but didn't know those in Ireland or Scotland were relatively small. Being they're further north, I figured they'd be fairly large for red deer standards.