The reason you have to kiss it upside down is because the actual point on the wall where you kiss the blarney stone is only accessible by leaning over backwards.
Imagine yourself on a flat roof of a stone tower. Enclosing the rooftop there's a small wall which is only maybe 3ft high, so you don't fall over.
Beyond that small wall is another wall of a neighbouring tower which is much taller, and although there's a sheer drop between the 2 towers, its actually only a couple of feet between them. The bit you want to kiss is on the taller tower, just below the level of the wall of the roof you are standing on.
Hence the only way you can reach that point with your lips is to lean over backwards (while someone holds your feet) and push your head back so your lips are close to the other wall.
I hope that explains it properly. It'd be much easier to describe if I could draw you a diagram.
