What literary device is used in the last verse of "Barbie Doll"?

Irony is the literary device used in Marge Piercy's "Barbie Doll" throughout the poem, and, certainly, in the last verse.

Irony is a literary tool that, unlike sarcasm and satire, is not cruel or biting; instead, it is a device that is subtle, and very effective in its delicacy. Futhermore, in "Barbie Doll," Piercy's subtle use of irony adds force to her poem, providing an extra dimension to her verse. For instance, the upbeat tone of the poem and the lilting rhythms ironically belie the tragic import of its message. 
When the girl is born "as usual," and she has the dolls and stoves and other things that most girls her age have, everything is normal. However, the verbal irony of " the magic of puberty," transforms her from a "usual" girl to one whose nose and legs are too large, and she is no longer "normal." 

Further, in the last verse, the irony is intensely effective as it points to how the girl has become so tortured that she disfigures herself and dies. It is only in death that she finally acquires a pretty turned-up nose, fashioned of putty by the undertaker.

Doesn't she look pretty? everyone said.
Consummation at last.
To every woman a happy ending.

Ironically, it is only in death--ironically termed "Consummation"--that the girl becomes "pretty" and acceptable in her appearance.

Without doubt, the poet hides her criticism of the demands of society by subtly attacking its artificiality of judging a person on physical appearance. She strikes at the damaging effects of such censure and criticism by using melodic rhythms and an ironic tone of formality and detachment.