Irony describes the contrast between expectation and reality as the speaker establishes it throughout the poem. The chief difference between what was expected and what really came to pass lies, of course, in the fate of the Titanic itself. Its makers and passengers expected it to cross the ocean with safety, speed, and comfort; in reality, it sank and killed hundreds.
The speaker brings out this larger irony through a series of smaller images focused on contrast. For example, stanza II features an image of the sea's currents flowing through the ship's boiler rooms. The image contrasts the water that now actually fills the "chambers" with the "salamandrine fires" that the ship's makers intended to fill the chambers. This contrast is made ironic because the opulent fireplaces now hold only the very element that puts fires out.
Stanza III offers an image of sea-worms crawling over mirrors. This juxtaposes the people who were expected to use the mirrors—the ship's "opulent" passengers—with the creatures who have actually ended up using thm, the "grotesque" worms. These images remind the reader of the vast difference between what the ship seemed to be—an unsinkable force, a human-made power greater than the power of nature—and what it actually turned out to be: something for the lowliest of creatures to crawl on.
Irony is also present in the poem's use of biblical allusions. For example, the terms "cleaving" and "mate" present the ship and the iceberg as parallel to Adam and Eve, who came together in the "consummation" of marriage just as the ship and iceberg come together in the "consummation" of a collision. But while God created Eve to help Adam and create life, the vague force (that "Immanent Will") in this poem "[p]repared" the iceberg to destroy the ship and cause death.
The phrase "stature, grace, and hue" and the term "consummation," create a parallel between the Titanic and Jesus Christ. But again, while Christ was sent by a compassionate God to save humanity, the ship is used by an indifferent deity/force to destroy humanity. It is ironic that the speaker would describe the wreck of the Titanic using the language and figures of the Bible when the emphasis in the Bible is on God's saving love for humankind and the poem depicts a supernatural power who plans and takes pleasure in humankind's destruction.
In describing the wreck from the perspective of this supernatural power, the speaker also adopts an ironic tone. Though describing a momentous tragedy, nothing in the speaker's tone suggests mourning or sorrow. Instead, like the "indifferent" sea-worms, the speaker is dispassionate. Some alliterated phrases ("Jewels in joy", "gilded gear") and the "praise" for the Titanic ("smart ship," "gaily great") even come across as outright mocking. The speaker's tone and implied attitude are ironic in that they violate the reader's expectations for how this tragic subject would be described.
Overall, the poem's irony functions both to chastise and to warn. The ship's makers seem even more at fault when their confidence is set against the ship's actual fate. And if this fate was planned by a supernatural power, the destructive indifference of this power is even more striking when contrasted with the sympathy of God.