Yeats devotes the entire third stanza of this poem to talking about a stone that's sitting at the bottom of a stream. But it becomes clear pretty quickly that he's comparing the stone to the people who have given their lives in the Easter Uprising. For Yeats, there's something interesting in the fact that the Irish fighters, like the stone, cannot be changed or moved even while the world changes around them. Their passion for Irish independence is like a stone, especially now that they're dead. They're removed from the world of change. Here, Yeats might actually be showing some shame for the fact that he's willing to go with the flow as far as the world's concerned.
Lines 41-44: Yeats suggests that the people who fought in the Easter Uprising did so with "one purpose alone / Through summer and winter." In other words, their sense of purpose did not change with the times. It remained constant and unmoving. Ultimately, Yeats compares this kind of unchanging dedication to a stone that remains unchanging in a "living stream."
Lines 55-56: After throwing down a bunch of descriptions of birds and horses running around the stream, Yeats reminds us that these things live "Minute by minute," but the stone stays the same "in the midst of all." Here, Yeats is comparing the dedication of the revolutionaries to the world around them that just keep changing with the times. But at the end of that day, the fighters have one more thing in common with the stone—they aren't alive. Again, Yeats pulls back from the impulse to totally celebrate these people as heroes.