The irony of loss
The primary irony of the book is the historical irony of Greek resistance at Thermopylae. Because of the strategic prowess of the Greek leaders, they were able to exploit the advantages of the Spartan elite (although they were only about three hundred in number) along with a natural advantage (luring the Persians into a narrow pass so they had to do man-to-man battle with the Spartans) to secure an important victory against the Persians, who had millions in their forces.
The ironic perspective
The story plays with perspective, offering an unlikely point of view, the Persian point of view. The novel shows Xerxes, guiding the war from afar, like a player at a chess table, receiving updates from messengers about the tides of battle. The perspective is also ironic dramatically, because Xerxes doesn't know until he is informed how the battle is actually going. He is at the whim of fate, in a way, but he doesn't know until he learns bit by bit what the reader knows from history; that three hundred Spartans are defeating Persians at an alarming rate.
The irony of nature
There is an important part in Sun Tzu's The Art of War where he warns military leaders that nature poses inherent risks, so the invading army should consider themselves at a natural disadvantage, because the home army will know by life experience what those natural advantages are. Xerxes learns this the hard way, through dramatic irony. The Persian army falls for a trap, being lured into a narrow pass that funnels their massive numbers into a battle with the Spartan elite. In the narrow space, the numerical advantage doesn't matter, because nature pushes them into man-to-man combat.
The detached leader
Xerxes is one of the world's most important leaders. He led the Persian empire into its expansion throughout the Mediterranean world. His power is ironic, because he is so powerful that his will can be accomplished at a distance. Now, he doesn't have to risk his own neck to do war; the empire is strong enough to sustain itself through administration of power. Although Xerxes is powerful, his power is slowed and detached from the heat of battle where his strategic feedback might have been more important.
Human life and loss
The novel observes an irony in the way human loss is perceived in battle. Remember in ancient literature that often, each death would be given a specific backstory; a stanza or two dedicated to each dying casualty. Well, when there are millions of people dying in a battle (although that number is probably inflated), those deaths become purely statistic; when the Spartans injure one of Xerxes's military leaders, he actually knows that man personally, and he extends his army to help that one man. Xerxes's army is too large for him to care about each person, but he does care about the people he knows.
