Lines 41-52 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Hearts with one purpose alone
Through summer and winter seem
Enchanted to a stone
To trouble the living stream.

Now Yeats is starting to put on his serious hat. In these lines, he compares hearts that have only one purpose only to a stone that splashes into or "troubles" the living stream of history.
Oh yeah, and we should mention here that Yeats isn't talking about hearts that have decided to rebel against their owners and jump out of people's chests. He's using a device called synecdoche to use people's hearts as symbols for the entire person.
So what he really means here is, "People who are devoted to a single purpose," kind of like the Irish fighters.
And on top of that, Yeats is comparing these people's "hearts" to a stone, which might sound kind of harsh at first. But he doesn't mean "cold and uncaring" when he talks about a stone here. He means that these people's sense of purpose is unmoveable and unchanging. Even though the seasons change like summer and winter, and the stream of life keeps moving and changing, these people's passion won't change even after they're dead.
Yeats' language is different here than earlier in the poem. By comparing the fighters to a rock, he's giving them a sort of respect that he hasn't earlier in the poem. Maybe he's finally starting to throw down some mad props for the dead.

The horse that comes from the road.
The rider, the birds that range
From cloud to tumbling cloud,
Minute by minute they change;

In order to show us just how unchanging the fighters' passion was, Yeats decides to throw down a bunch of nature imagery to give us an example of things that actually do change over time.
For example, someone riding along the road on a horse changes; the birds that fly above him change and the clouds keep floating along. "Minute by minute," all of these things change.
But not the passion of the fighters who have died. They can't change anymore because they're dead, and they died with thoughts of a free Ireland in their hearts, which makes these passions stay the same forever.
Pretty intense, eh? Yeats is getting farther and farther away from the snobby tone he started this poem off with.

A shadow of cloud on the stream
Changes minute by minute;
A horse-hoof slides on the brim,
And a horse plashes within it;

Hooray. More nature imagery. Nice clouds that get reflected in the moving river. Nice horsey sliding into the water and splashing around.
But this is all going on in a world where the people Yeats is remembering are dead and gone. The world continues on without them, it seems. But we should never forget here that these people and their political cause still exists like an unchanging stone at the bottom of the stream.
It's a metaphor for how there's something permanent left behind by these people's sacrifice, even though the world might go on changing with the seasons. You might not be able to see their lingering presence; but it's still totally there.