"Easter 1916" by William Butler Yeats is a poem responding to the "Easter Rising," an event in the long struggle by the Irish for independence from England that took place in April 1916. In the decades immediately preceding the Easter Rising, this struggle had been mainly pursued through legal avenues and peaceful protests, but as conflicts between those in favor of Home Rule and Unionists intensified, Ireland moved to the brink of civil war.
Yeats's poem shows a gradual evolution in Irish life before the Easter Rising. It describes the Republican leaders having ordinary lives, with Constance Markievicz enjoying the relaxing leisure pursuits of an aristocratic woman (hunting rabbits on horseback) and Patrick Pearse and Thomas MacDonough portrayed as poets pursuing their art. The intensification of the political conflict transforms them, with Markievicz becoming argumentative and "shrill" rather than merely being decorative and feminine (a change of which the narrator disapproves) and the men also being "changed utterly" from people involved in ordinary life to something both noble and terrifying.
The third stanza suggests Ireland itself was seen as having a certain tranquil pastoral beauty which the rebellion changed, just as a horse and rider can scare away the birds floating peacefully on a stream.