In addition to the rhythm of the lines established by blank verse (which we discuss at length in "Form and Meter") this poem has tons of impressive sound effects—even Timbaland would be impressed. Heaney is really trying to make the sounds of the poem reflect the sounds of the place. So think swampy suction sounds, and hopping frogs, and you've got what he's after. When the tone in the poem shifts from enthusiastic curiosity to horror and disgust, the sounds shift too. Let's take a look at some of the tools he uses most often.
Right from the get-go Heaney hits us with a healthy heaping of alliteration (yes, we did that on purpose). In line 1, you hear the repetition of the Fsound: "the flax-dam festered," and then quickly in lines 1 and 2 you hear the repetition of the H sound: "heart/ Of the townland; green and heavy-headed." It continues throughout the poem—"jampotfuls of the jellied"—and so forth and so on.
Similar to alliteration are consonance (close repetition of similar or the same consonant sounds), and assonance (close repetition of similar or the same vowel sounds). Heaney uses consonance most forcefully when the tone of the poem has shifted, and all of a sudden this natural world feels threatening to our speaker. The harsh consonants help hammer home a sense of violence and threat that coincide with what the speaker is experiencing. Take a look at the repetition of hard C sounds throughout the second stanza when things start to go sour: "rank / With cowdung [...] I ducked through hedges / To a coarse croaking" (22-25). And a bit further on: "bass chorus […] were cocked […] I sickened, turned, and ran [...] The great slime kings […] would clutch it" (26-33).
Assonance is a bit subtler and helps braid the sound of the poem together without us even noticing it. Heaney gets to it right away, with the long A sound in: "heart […] heavy headed […] weighted" and just a little later with the short U: "punishing sun" and "bubbles." Heaney uses assonance as back-up vocals to the more forceful alliteration and consonance. You don't really notice it when you're listening, but the song's way better because it's there.
One of the coolest thing Heaney does with sound is onomatopoeia, which is when a word sounds like what it's trying to represent. For example, the speaker encounters the frogs in the dam and they're being all nasty and threatening:
[…] gross-bellied frogs were cocked
On sods; their loose necks pulsed like sails. Some hopped:
The slap and plop were obscene threats. Some sat
Poised like mud grenades, their blunt heads farting. (27-30)
This really sounds like what it is. "Slap" and "plop" sure does sound like frogs jumping around. Heaney knows this and uses onomatopoeia to add texture to the scene and draw us in.