I would say that you should go to Kilternan. If you don't go and get the basics of putting on skis, standing up, moving a little, getting control, etc . . . then you run the significant possibility of spending the first 2/3/4 days of your expensive ski-school time on your expensive ski holiday standing around, getting cold, watching everyone else go up the slopes while you fall over and fall over and fall over on the nursery slopes.
When you start in ski-school the instructors will normally get you to do a short ski (like 10-20 metres) to show whether you have any control at all. If you have no control (because you have no experience at all) then you *will* be in the absolute beginners class and unfortunately this class will go at the speed of the slowest in your group. You may be lucky and all beginners in your group are quick learners or you may be really unlucky. If you are in the beginners class there is nowhere lower for a slow skier to go, so your instructor must stick with them.
It is expensive enough in Kilternan, so it is worth considering whether you want to pay that much to get the advantage, but if you are paying a lot of money for the holiday you may want to maximise the actual slope time you get by getting the basic learning out of the way.
I agree that the slopes in Kilternan hurt a lot when you fall and that the sensation of skiing there is nothing at all like skiing on snow, but you get the control basics there and that is worth it.
z