It's not like Yeats' language in this poem is all that tough. But what is tough is the kind of thinking that Yeats is trying to capture. Basically, he wants to commemorate people who died in the Irish Uprising, but doesn't really know if the Uprising was all that great an idea. The only thing he can say to express his contradictory feelings is that the whole thing seems "terribly beautiful" to him in a weird way. Because Yeats can't really figure out how he feels, it makes it tough to follow the lines of thinking in this poem.
Plus, it's tricky to understand all those references if you're not an Irish person from the early 20th century. Good thing Shmoop's here to help.