This is a good point, Peter. It's true that the meter relaxes at exactly the most late-19th-century, early-Yeats-ish language and material-- almost as though he's making the phrase "Celtic Twilight" a literal, rather than literary phrase!-- and that relaxing doubtless is part of my distinct memory of finding that I had all of the poem by memory except for these lines.

I'm not sure what to make of what you point out. But it is interesting. (And incidentally, convincing detective work on your part.)