When the speaker says he wishes to be "set upon a golden bough to sing," he is talking about his desire to be a mechanical bird. All through the poem, the elderly speaker has been lamenting his aging, saying that the world is not made for old men like him. Being old, he says, is like being "a tattered coat upon a stick" unless his "soul" can drown out the effects of aging.

Therefore, he decides in imagination that he is sailing to a mythic Byzantium, a "holy city." In Byzantium, he wants to give up his dying body and replace it with one crafted by an artisan. As a work of art, he will be able to avoid dying and live forever. He dreams of being a beautiful bird crafted out of gold, which can sing for the emperor. As a golden or artificial bird, he will naturally be set on an artificial golden branch to sing his mechanized song into all eternity.

The poem is a lament for the fact that all humans grow, age, and die. The speaker wishes to trade that state for the timeless beauty of becoming a work of art. This is a wish and dream more than a reality, a happy place the speaker can use as an escape from the inevitability of death.