How do the multiple speakers in the poem "Ozymandias" help create a sense of irony?

You could say that there are three speakers in this poem even though there is only one speaker relating what has been said. The main speaker of the poem recalls what a traveler from an ancient land had told him about the statue. The irony arises between what the traveler says about the statue and what Ozymandias "said" (the quote) to all who would look upon his statue. 

Ozymandias is quoted as saying "Look on...

You could say that there are three speakers in this poem even though there is only one speaker relating what has been said. The main speaker of the poem recalls what a traveler from an ancient land had told him about the statue. The irony arises between what the traveler says about the statue and what Ozymandias "said" (the quote) to all who would look upon his statue. 

Ozymandias is quoted as saying "Look on my works, ye Mighty, and despair!" Ozymandias (Ramses II) was quite proud and wanted monuments such as this statue to demonstrate his greatness and superiority to all others. However, in an ironic twist, the sculptor is able to mock this pride by capturing and mocking Ozymandias. The sculptor gives him a "wrinkled lip, and sneer of cold command." Ozymandias wanted the sculptor to illustrate his greatness, but the sculptor mocks the ruler's pride instead. 

The further irony is that the statue has become a "colossal wreck" in a barren desert. Contrary to Ozymandias's wish, his greatness has been forgotten. 

The speaker of the poem reflects upon this notion that power is fleeting, but that the art (sculptor's work) has lasted. It is the mockery in the art, not the ruler's greatness, that has survived the test of time. Art outlasts power.