Irony of understanding
Although we cannot understand some of the language of this poem, Carroll uses certain sounds to give us an idea of what he is talking about. For example, in the first stanza, "slithy" sounds a lot like "slimy."
Irony of mystery
Although this poem is often thought to be impossible to understand, Carroll actually gives a hint of what the first stanza means via the character of Humpty-Dumpty in Through the Looking-Glass. The character says what some of the words mean, for example "brillig" means four o'clock in the afternoon.
Irony of the Jabberwocky
Although the Jabberwocky is depicted as a frightening and formidable creature, he is killed very easily by the protagonist.