"Easter 1916" is Yeats' grim response to what is known as the Easter Uprising of 1916, which was centered in Dublin, Ireland, on the morning after Easter. Although the rebels opposing British rule in Ireland felt that they had a chance of success, most historians believe the rebellion had no chance of success other than to momentarily frighten British authorities. The timing of the rebellion has often been attributed to the possibility that the rebels believed that Irish citizens would join the rebellion once it started; and the rebels may have believed that the British, completely engaged in WWI in Europe and elsewhere, wouldn't have the will to fight Irish independence.
In any case, about a thousand rebels participated in the beginning, taking over the General Post Office but, unlike the British troops stationed in Dublin, the rebels had only rifles and a few hand grenades whereas the British had rifles, hand grenades, machine guns and small artillery pieces. The rebels, however, took over several major buildings in broad daylight to encourage others to support them.
On Wednesday, the British began their assault on the rebel troops, who initially believed the British would try to spare both civilians and buildings. Unfortunately, the British were intent on destroying the rebellion as quickly as possible and with as much force as possible, so they fired on and destroyed (if they had to) any building being used by rebel troops for cover. In addition, because the British couldn't tell combatants from non-combatants, they assumed any civilian in the city was a rebel and acted accordingly.
The rebels surrendered on Saturday, and the leaders were executed almost immediately. The Irish public was horrified by the callousness with which the British handled the prisoners and executions, and the irony is that although the Irish did not support the rebellion while it was going on, they began to support the rebellion and its "martyrs" after the fact, which ultimately helped lead to the Irish Civil War.