The poem packs a striking amount of repetition—especially parallel phrasing and repeated images—into just 24 lines. This effect adds to the hypnotic quality of the verse and the spellbinding quality of the tale it tells.
In the first stanza alone, three major images are mentioned twice:
I went out to the hazel wood,
[...]
And cut and peeled a hazel wand,
And hooked a berry to a thread;
And when white moths were on the wing,
And moth-like stars were flickering out,
I dropped the berry in a stream
Combined with the steady rhythm of the verse and the anaphora of "And" at the beginning of lines 3-6, the repetition of "hazel," "berry," and "moth[s]" makes this "Song" sound almost like an incantation or spell. Even the apparently singular word "fire" in the second line will soon return in the second line of the next stanza.
The second stanza contains several similar repetitions: "floor" in lines 9 and 11 (also an identical rhyme), "called me by my name" in lines 12 and 15, and "something"/"someone" in lines 11 and 12. (Here, the shift from something to someone indicates the fish's transformation into a human.)
Along with more anaphora of the word "And" in lines 20-22, the third stanza contains two mentions of "lands," the parallel phrases "kiss her lips" and "take her hands," the near-identical "time" and "times," and the parallel phrasing "The ___ apples of the ___" in the final lines. ("Apples" also echoes the "apple blossom" in line 14, while "silver" in line 23 echoes "silver" in line 8.)
The speaker seems almost compulsively drawn to pairings of words and phrases—fittingly enough, since he hopes to find and pair off with the girl of his dreams. Ironically, though, in this poem full of pairs, Aengus remains alone!