The poem alludes to the lives of sisters Eva Gore-Booth and Constance Markievicz, who grew up at Lissadell House, an Irish estate built during the Georgian era. Yeats sometimes vacationed there, and he admired both the beauty and courage of the sisters, whom he recalls wearing "silk kimonos" and looking "Beautiful." Gore-Booth, Markievicz, and Yeats shared a desire for a free, self-governed Ireland as well as an Irish culture (and literature) separate from Britain, which had colonized Ireland in the 16th and 17th centuries.
Indeed, as the first stanza recounts, "The older" sister, Markievicz, was "condemned to death" for her part in the Easter Rising of 1916, an armed insurrection by the Irish against British rule in Ireland. She was later "Pardoned," and went on to help establish a democratic political party in a newly independent Ireland. Yeats' remark that she spent the latter "lonely years" of her life "Conspiring among the ignorant" says more about his own disdain for democracy than it does Markievicz, who is largely remembered for her part in helping to establish an Irish Republic as well as her work as a suffragist and labor rights activist.
Yeats also disparages Gore-Booth's trajectory from the beautiful, inspiring young woman he remembers to someone "withered old and skeleton-gaunt" chasing after "Some vague Utopia." Gore-Booth was also associated with the Irish Nationalist cause, but she, too, sought a more democratic approach to freedom and fought for women's right to vote and work. She was also an animal rights activist, a member of the Women's Peace Crusade, and a poet.
While Yeats clearly disagreed with the sisters' politics—outside of their shared passion for an autonomous and culturally distinct Ireland—he admits that their differences in opinion feel silly now that the sisters have died. Regardless of who was "wrong or right" about what, he believes that together they "built" a "great gazebo": a metaphor for the independent Ireland they fought so hard to bring about.