All changed, changed utterly A terrible beauty is born (15-16)
For most of his adult life, Yeats was happy to walk around Dublin and make meaningless small talk with the people around him. But all of that changed with the Easter Uprising of 1916. After that, everything was different. A lot of people were dead, yet Yeats can't help but feel that their historical sacrifice has a sort of terrible beauty to it.
He, too, has been changed in his turn (38)
Yeats wasn't all that big a fan of Major John MacBride. But he still has to admit that the dude has become a little bit immortal because of the role he played in the Easter Uprising. Then again, the phrase "changed utterly" might just mean that MacBride is dead. You never know with Yeats.
Hearts with one purpose alone Through summer and winter seem Enchanted to a stone (41-43)
For Yeats, there's something potentially immortal in the fact that the Irish fighters had a passion for freedom that never seemed to change over time. In this world, most things do change like the seasons. But the fighters' passion is immortal like a stone, unchanging and permanent.
Minute by minute they live: The stone's in the midst of all (55-56)
Things in this world tend to change minute by minute. We get up, we move around, and life goes on. But the people who've died in the Easter Uprising can't change anymore because they're dead. On the other hand, there's almost something immortal about the fact that there's no more changes left for them to undergo.
No, no, not night but death (66)
For the past few lines, Yeats has been talking about how "night" has come over the people who fought in the Easter Uprising. Now usually, most poets would be content to leave the metaphorical connection between night and death up to us. But not Yeats. Here, he decides to make it clear that he's talking about death. As in dead. No immortality, despite what he's said in other places.
Now and in time to be, Wherever green is worn (77-78)
At the end of the poem, Yeats decides that after all of his hemming and hawing, he thinks that the people who've died in the Easter Uprising have achieved some sort of immortality. After all, these people are part of Irish history, and whether people know their names or not, they'll always be part of what's going on whenever green is worn. That's because green is the color of Ireland and these people died for the cause of Irish freedom.