Iambic Tetrameter and Trimeter
For the most part, Yeatsy likes to stick to iambic tetrameter with an ABAB rhyme scheme, which is pretty much as straightforward as it gets for meter and rhyme. But every now and then he'll slip into some trimeter.
For example, you can see the switch from tetrameter to trimeter in lines 5-6, where Yeats writes, "I have passed with a nod of the head/ Or polite meaningless words." The sudden switch to trimeter helps convey the idea that Yeats like to keep these superficial meetings with people nice and short. But he suddenly switches this tendency in line 15, where he uses trimeter in the phrase, "All changed, changed utterly." So his superficial world has suddenly changed into something way more serious—a world of death and executions.
Yeah, Yeats kind of knows what he's doing with all this meter stuff. Dude wasn't a master poet for nothin'.