A Half-Remembered Poem

I memorized a poem last summer. Now I can only remember fragments.

The third stanza talks about travelling to (some mythical city) as an allegory of death.

When he gets there the narrator will encounter golden mosaics. He will “dance in the fire, pyrn in a gyr.” Since I cannot spell these words I cannot google them.

Then he will take a shape of his own choosing, a bird perched in a tree. In the last line he will sing to the lords and ladies of the city of what is past, present and yet to come.

Any idea what I am talking about?

Sailing to Byzantium

A real city, BTW.

Thank you. A good funeral poem.