What could have made her peaceful with a mind That nobleness made simple as a fire, (lines 6-7)
Modern people are messy and complicated; we tend to think of the ancients as being simpler and more consistent. Maud Gonne's passion is simple and pure, like a flame. Yeats is laying the groundwork to compare her with Helen of Troy.
With beauty like a tightened bow, a kind That is not natural in an age like this, Being high and solitary and most stern? (lines 8-10)
Gonne is literally a woman from another age, a blast from the past. Her appearance – and also her character – seem straight out of a culture that uses bows and arrows...Hmm, we wonder what culture that might be...
Was there another Troy for her to burn? (line 12)
Ah, yes, Yeats always has to bring the Greeks into it. Maud Gonne is suddenly blended together with Helen of Troy, to form a part-past, part-present superheroine! Following a long tradition in Western literature, Yeats suggests that Helen was largely responsible for the Trojan War by cheating on her husband with a bratty Trojan prince.