Irony is a common way of achieving tone, and is done when two ideas or images are put together that would seem more naturally separate. For instance, in the phrase, "Moving from Cheer to Joy, from Joy to All, I take a box."
Irony is created in that phrase by placing words normally used to describe emotion alongside the word "box." The poet is writing about an experience in a grocery store, and the words are not in fact words for emotion but, in fact, brands of laundry detergent.
A further irony is created by the knowledge that the poet is a man writing in the 1950s, and would therefore seem out of place in a grocery store.
Rhythm
Rhythm is a classic component of poetry, and there are specific rules.
Poetry scholars who analyze rhythm divide the lines of a poem into sections called feet and classify them in the following manner:
Monometer: one foot
Dimeter: two feet
Trimeter: three feet
Tetrameter: four feet
Pentameter: five feet
Hexameter: six feet
Heptameter: seven feet
Octameter: eight feet
Once the number of feet is determined, a poetry scholar then observes the syllabic structure within each foot and classifies them in the following manner:
Iambic: a short-stressed syllable followed by a long-stressed syllable. For example, words like "indeed," "about," or "against."
Trocheeic: a long-stressed syllable followed by a short-stressed syllable. For example, words like "certain," "women," or "patient."
Dactylic: a long-stressed syllable followed by two short-stressed syllables. For example, "muttering," "restaurants," or "oyster-shells."
Anapestic: Two short-stressed syllables followed by a long-stressed syllable. For example, "afternoon," "do I dare," or "overwhelm."
Spondeeic: A long-stressed syllable followed by another long-stressed syllable. For example, "one night," or "shirt sleeves."
So, for instance, in the phrase, "Let us go then you and I," the rhythm scheme would be trocheeic tetrameter.