What symbolism is in "Easter 1916"?

An early symbol in the poem appears in the first stanza: it is "the fire at the club." This represents the comfortable middle-class world people like him live in, with their "polite meaningless words" and genteel eighteenth-century houses. This world and its occupants are a contrast to the Irish freedom fighters—whose sacrifice causes a terrible and beautiful world to be born.
An important symbol in the poem is the stone in the "midst" of the stream that appears in the second stanza. This stone represents the Irish freedom fighters, unmoved by the changing times: steadfast and purposeful. They are directly contrasted to images that symbolize fleeting, transitory moments, such as clouds and horses racing by. While the stone stays the same, these other symbols are shown in all their changeableness. The clouds, for example, are first "tumbling" clouds and then appear as the "shadow" of a cloud reflected in the stream.

The clearest examples of symbolism in W. B. Yeats's poem can be found in the final two stanzas. Yeats uses natural imagery and an extended simile comparing "hearts with one purpose alone" to a stone which "trouble[s] the living stream," or the onward passage of time and the natural order of things. The stone symbolizes the stony resolve of these people who are so dedicated to the cause of Irish independence, but its position at the bottom of the "living stream" also represents the fact that these people are now removed from the action of what is happening. The next stanza, which references several heroes of the Irish independence movement who lost their lives to it, makes clear that this unmoving stone "in the midst of all" that still lives "minute by minute" symbolizes these dead soldiers, wedded to their purpose even in death. They are, after all, soldiers of "summer and winter" (compare Thomas Paine's "the winter soldier" and "the summer soldier") and their hearts have become stones wedded to their purpose through "sacrifice."
Another, more straightforward instance of symbolism can be found in the final stanza describing "wherever green is worn." Green is a color strongly associated with Ireland and here symbolizes Ireland and the Irish.