Reached Irony

Cassia taking the red tablet (Dramatic Irony)

Cassia's lack of immunity to the effects of the red tablet results in the reader being privy to an entire chapter's worth (Chapter 5) of events which she cannot remember.

Xander not realizing that the blue tablets are poisonous (Dramatic Irony)

Another example of dramatic irony comes from Xander being ignorant of the fact that blue tablets are poisonous (or, more accurately, that they awaken a dormant form of the Plague within the consumer, which we don't learn until late in the book). Though Ky and Indie have known the true nature of the blue tablets for some time, Cassia was also unaware of it until the middle of Crossed, and it's only natural that Xander, too, remained in the dark for a significant portion of Reached.

Xander being punished for carrying out Oker's wishes (Situational Irony)

A perfect bout of bad luck implicates Xander in the destruction of the camassia cures in Chapter 43, as well as suspiciously links him to Oker's death in the same scene. While he was only trying to do as Oker suddenly and rather surprisingly instructed, he ultimately appears to have maliciously sabotaged the cures, which could not be further from the truth.

Cassia Matching herself with Ky (Situational Irony)

One of the biggest questions throughout the Matched trilogy was who put Ky's name in the Matching pool, causing Cassia mistakenly to receive two Matches. That it was actually Cassia all along who did it without remembering is perhaps the most poignant example of situational irony that Reached has to offer.