Son of the Shadows question

Why do you assume the child was not Liadan? From what I understood in Son of the Shadow, Liadan's birth along with Sean was a surprise, it was not predicted. I think this is the reason the third child was shadowy in Sorcha's vision. Liadan was a wild card character. Remember Liadan disobeyed the Fair Folk about taking the child away from the forest and broke the pattern they were trying to make. So if Liadan was not supposed to exist what of Niamh's affair with Ciarin. Was this part of the Fair Folk's plotting to produce the Son of the Shadow? Was Niamh intended to marry the O'Neil (not the spelling, I know) chieftain and produce a son? But how would that be a Son of the Shadow? Actually I think the Son of the Shadow was not in the original prediction, that it only became so with Liadan's existence (hurray for at least partial free will). It is very convoluted.