The poem
A LATE lark twitters from the quiet skies: And from the west, Where the sun, his day's work ended, Lingers as in content, There falls on the old, gray city An influence luminous and serene, A shining peace.
The smoke ascends In a rosy-and-golden haze. The spires Shine and are changed. In the valley Shadows rise. The lark sings on. The sun, Closing his benediction, Sinks, and the darkening air Thrills with a sense of the triumphing night-- Night with her train of stars And her great gift of sleep.
So be my passing! My task accomplish'd and the long day done, My wages taken, and in my heart Some late lark singing, Let me be gather'd to the quiet west, The sundown splendid and serene, Death.

I searched all over just to know what MARGARITAE SORORI means then i found out it was italian for SISTER MARGARET. since this is William Ernest Henley's dying poem then he must be referring to Saint Margaret of England, right? because Saint Margaret is the patron saint od the dying.
United States
Actually, "Margaritae Sorori" means "to sister Margaret". The "to" is very important because the title is a dedication. The poem is actually dedicated to his little daughter Margaret who died, at age 6, from tuberculosis. She is Henley's "sister" because we are all brothers and sisters in death, but also because Henley suffered (and knew he would eventually die) from TB.