Features of modernism that are expressed in Yeats's poem are cynicism about modern society as well as interest in different experiences of time.
Yeats's speaker opens the poem by showing the banality of everyday life in 1916, a place of "grey" eighteenth-century houses, where people work behind "counters" or at desks. The speaker says more than once that he exchanges "polite meaningless words" with these fellow citizens, the repetition of the phrase emphasizing the meaninglessness of contemporary life that the modernist poets often noted.
The way these ordinary people experience time, the speaker says, is like
a shadow of cloud on the stream
[that] Changes minute by minute.
They live entirely in the flux and churn of the modern world, which has no more lasting solidity or meaning than the reflection of a cloud on water. But the four martyrs to the cause of Irish independence, who the speaker celebrates, exist in a different kind of time, one that, unlike an ever-changing cloud, resembles an unmoving, unchanging "stone." By standing on timeless, universal principles to fight the British, these four ordinary people are "transformed utterly." They enter a different kind of time, one that is not concerned with mortality. Out of this,
all [is] changed, changed utterly
a terrible beauty is born.
This attempt to capture the idea that not all time is the same was a preoccupation with many modernist writers, who felt that reality cannot be captured without acknowledging differing perceptions of time, including the kind of mythic timelessness that Easter represents.
While Yeats applauds and honors the heroism of these rebels, his modernist sensibilities reign in his enthusiasm with a degree of cynicism. With World War I raging in the background of everyone's minds, a pointless bloodbath, he asks of these people,
Was it needless death after all?
This edge of despair puts Yeats in the modernist camp, questioning values that the Victorians might have accepted with glorious optimism.