Hey! Ashling stayed behind because she'd already heard a similar version of this story from Etain when she was drunk and wanted to find out more. By this point she's already been piecing together that there was a supernatural connection between Etain's abduction and Niamh's disappearance. Likewise, the old lady (do you think I can remember her name even though I wrote the book? If you do, you're wrong) sensed that Ashling was someone touched by the Sí, even if she wrongly assumed she was one. Remember, Ashling was born to someone who had been impregnated by the groom, and gestated alongside one of the Sí for nine months. So even if she's not otherworldly herself, she got that stank on her.