You clip for a purpose to make the horse more comfortable when he has a thick coat - he is not a pair of designer jeans!
It may seem to be fun but you need to think of the horses comfort - too many people clip crazy designs on their horse that leave vast areas of body uncovered that needs to be warm.
Unless you have someone running around after you with a blanket to throw over the horses back when waiting around at shows the poor horse is left with a cold back and cold muscles get damaged easily.
Would using chalk to help outline the shamrock help to clip the pattern in?
What about brushing the hair in the wrong way with the stencil on, so that you can still see the shamrock, but you aren't clipping the summer coat? Or using a product like Twinkle Horse glitter in green?