Being certain that they and I But lived where motley is worn (13-14)
Yeats always lived his daily life figuring that he lived in a world where nothing should be taken too seriously. He was totally sure that that was the basic principle of daily life—nothing really matters too much and everyone is more or less satisfied with the way things are. But Yeats turns out to be wrong on this one.
All changed, changed utterly A terrible beauty is born (15-16)
As a matter of principle, Yeats feels like there's something important that has happened with the Easter Uprising. He wants to connect all of the death and sacrifice to larger events that have taken place throughout history. And even though all the death might be terrible, there is something beautiful about how it fits in with the cycle of death and violence that has always been going on in the world.
Her nights in argument Until her voice grew shrill (19-20)
It looks like Yeats isn't all that big a fan of women entering the world of politics. While he tends to be pretty nice to the men involved in the Easter Uprising, Yeats regrets the fact that a beautiful woman like the Countess Constance Markievicz threw away her femininity by yelling about politics until her voice "grew shrill." Sheesh, Yeats. We know you wrote a long time ago, but that's still pretty misogynistic.
He, too, has been changed in his turn (38)
Even though Yeats could never stand Major John MacBride, he is willing to give the guy his due for sticking to his principles and fighting in the Easter Uprising. John MacBride was the husband of a woman Yeats was totally in love with, so you can see how giving MacBride his props would have been tough for Yeats.
Minute by minute they live: The stone's in the midst of all (55-56)
The world tends to be a place where things are constantly changing and people are adapting to that change. But like a stone, the fighters of the Easter Uprising weren't willing to take things as they came. They decided to take matters into their own hands. Their unchanging principles make them like a stone that won't change even though water might flow around it.
That is heaven's part, our part To murmur name upon name, As a mother names her child (60-62)
It turns out that it might not be our role as humans to decide when there's been enough death and suffering in the world. According to Yeats, that's the responsibility of heaven or God. It's our responsibility to just keep having babies and mourning each other when we die. Yeesh, no wonder the guy didn't bother fighting in any battles. That's about as stoic as it gets.