David's random death
When David dies, it's easy as the reader to say, "Oh, right, because of the wars." But that's not what the girls in the story felt. To them, the war came suddenly and the death of their brother was something they could not have fathomed even a few months before it happened. Therefore, there is dramatic irony restricting the characters from understanding their horror in light of the greater horror of Europe at war.
The ironic sister
What should have been regarded as a cute irony turns into a massive issue when Emmeline can't see that the real reason Hannah seemed unimpressed by Robbie as children was because that's how she was showing affection those days, as a child, as a girl. Now trapped in a loveless marriage, Hannah loves Robbie deeply, even though Emmeline suspected Robbie and her would be the item.
The gun
The gun is an irony symbol because it represents Emmeline's willingness to be dramatic, but not in the presence of Robbie himself. What was designed as a desperate stunt quickly turned into a lifelong mistake when Robbie interprets the gun in light of his training as a soldier in the war.
The irony of shell-shock
Shell shock is when a bomb goes off nearby and a soldier is left in a state of daze and confusion following the concussion. The term was used to describe the unique affliction facing the WWI veterans returning to war, because it seemed that their mental states were permanently shell-shocked. The irony of course is that the characters in the book haven't figured out the truth about PTSD yet, so the men in the story are perplexed by constant terror and pain.
The star-crossed marriage
The odds of something going wrong for Robbie and Hannah were inevitable. Hannah is abandoning her family, for one thing, and Robbie isn't exactly all put together. This seems like Romeo and Juliet, but the irony of course is that in this story, Hannah herself kills Robbie.
