Away With The Fairies Meaning

(idiomatic) To be not not all there, to be slightly crazy.

Example: 2000, Peter Watt, Cry of the Curlew,
  Now, I know the story is away with the fairies, but he produces a body and I take him down to the station to talk to the detectives.
2001, Finola Moorhead, Darkness More Visible,
  That was so ridiculous I laughed. A smile would crack Alison's poetic persona so she looked serious. She put her thin hand to her cheek like a lady writer in a photograph with eyes that could penetrate surfaces. She was certainly away with the fairies.
2005, Ben Wright and Michael Patrick Shiels Good Bounces and Bad Lies,
  Hobday was wild, mad, and totally away with the fairies.