To me, the general irony in the poem lies in the simultaneous praise and scorn of this "Negro in Harlem." Written in 2nd person, the speaker communicates directly with an unnamed black man or woman, assumedly walking down the streets of New York during the Harlem Renaissance. Words like "magnificent," "perfect body," and "splendid" praise this person's natural beauty. "Your head thrown back in rich, barbaric song" and "I love your laughter," draw from more primitive African images and praise this person's pride in identity. In one reading, it sounds very much like a thematically accurate poem for the Harlem Renaissance. It sounds like a celebration of individuality and heritage, and encouragement to stand tall and proud.
However, mixed with these images of praise are negative images and negatively charged diction which suggest a tone of scorn. The phrases "Pompous gait," and "Small wonder that you are incompetent to imitate those whom you so despise," have a slight suggestion of something negative.
Don't read it wrong, however. The speaker does not scorn the individual of whom he speaks. Instead, the negatively charged diction plants the already present seed of scorn which existed between black and white people during that time. Black Americans were struggling with the balance between being fully themselves and loving who they were, and being hated and scorned for exactly that by white people.
This poem manages to capture both the black and white sentiments together. The irony lies in the blending of these sentiments. It is sadly ironic, afterall, that the very thing that makes some people who they are, is what makes others hate them.