Tichborne was executed for having become a party to a Catholic plot to murder Elizabeth. Although not quite as young as his farewell poem suggests, written within three days of his death, the verses were famous in the period.
Brown, Nancy Pollard. "Southwell, Robert [St Robert Southwell] (1561–1595)." Oxford Dictionary of National Biography. Ed. H. C. G. Matthew and Brian Harrison. Oxford: OUP, 2004. Online ed. Ed. Lawrence Goldman. Jan. 2008.