Answer:
It is ironic because the poem is celebrating the victory of the civil war but also mourning the death of the captain, President Lincoln.
Step-by-step explanation:
Irony is when there is a contrast between what is happening and what is expected. This means when the reality and what is thought of are in complete contrast, then there is irony.
Walt Whitman's poem "O Captain! My Captain!" is an elegy about the death of President Abraham Lincoln. And the irony in the poem is that it is a celebration of the end of the civil war and that they had won. But at the same time, the captain is dead, leaving them heartbroken. These two feelings of triumph and defeat at the same time are what make it an irony.
Celebrating the victory, the poet proclaims "the prize we sought is won, The port is near, the bells I hear, the people all exulting" but at the same time, "[the] Captain lies, Fallen cold and dead."