Photographs
Photographs are almost always perceived as a depiction of the absolute truth, a frozen image of a moment in time preserved exactly as it was. Photographs are meant to help solve mysteries. It is ironic, then, that for Jacob, seeing the photographs of the peculiar children only makes him more confused because he believes they have been manipulated.
Dr. Golan
The revelation that Dr. Golan was actually a wight seeking to manipulate Jacob is ironic. A psychiatrist is meant to be trusted absolutely with a person's fragile mind and mental conditions, and yet all the while Dr. Golan was messing with Jacob's head, convincing him that he was crazy when really he knew that what Jacob saw was the truth.
The Hollows
The existence of the hollowgasts is ironic in itself. As peculiars, the hollows sought out immortality. In a way, they got it because as monsters the hollows can live indefinitely. However, they live a soulless life as a monster, undoubtedly not the kind of life any of them envisioned for themselves.
