(Pedantic mode on)
according to AA route-planner:
Shannon Airport to Cork = 78 miles
Waterford Airport to Cork = 79.5 miles
... so no actual saving in distance there.
However (pedantic mode off) the Cork to Waterford road is probably a better road for most of the journey, and provides more opportunities for overtaking that feckin tractor that just pulled out in front of you and ain't going nowhere fast for another 20 miles.