Thus ceas'd she: and the mountain shepherds came, Their garlands sere, their magic mantles rent; The Pilgrim of Eternity, whose fame Over his living head like Heaven is bent, An early but enduring monument, Came, veiling all the lightnings of his song In sorrow; from her wilds Ierne sent The sweetest lyrist of her saddest wrong, And Love taught Grief to fall like music from his tongue.
Who is "she" and why is she "ceas[ing]" (stopping)? "She" could be Urania, or she could be Death. Let's see who else appears in the stanza for some clues…
First up, we have the "Pilgrim of Eternity," which refers to Lord Byron, famous poet (and noted bad boy of his time). He's famous, though his fame is as bent as a halo would be (probably because of his reputation), were he to have one.
Then, Ierne, a goddess from Irish lore, sends her finest "lyrist" (lyricist) to pay respects. Here, Shelley is referring to Thomas Moore, an Irish poet. His love for Keats makes poetry fall "like music from his tongue."
Our speaker says that the finest poets will write great works in honor of Adonais-Keats. We guess that's some consolation.
We still don't know who "she" is, though. Whether she's Urania or some other mourner, we can be sure of one thing: she doesn't mourn alone.