The quatrains with regular rhyme (both end and slant) make for built-in sound effects. Heaney also establishes a predictable rhythm, too (even if the meter is not consistent), with lines that are consistently 8 or 9 syllables. If you think about the action in the poem—the way the father plows up and down each row—you'll see how the methodical action really corresponds with the predictability of the lines and stanzas in the poem. And that's no coincidence at the hands of sound master Heaney.
Though the rhyme is probably what stands out the most in terms of sound, Heaney is using some other elements of sound, especially at the beginning of the poem, to draw you into the poem. It's practically like sonic hypnosis. Listen closely (you're getting very sleepy…).
In the first stanza and even trickling into the second, we get something called sibilance, which is the close repetition of S sounds. For example, in the first stanza you'll see: "horse," "his shoulders," "sail strung," and "horse strained." All those S sounds really swish us along, maybe trying to mimic the smoothness with which the father is able to plow because he is so skilled.
You'll also notice lots of consonance (the close repetition of consonant sounds) too, especially in the first couple of stanzas: "clicking," "sock," "breaking," "pluck." The hard K sounds really stand out, as if Heaney is using them to convey the hard nature of the work.
The smooth and steady sound Heaney achieves with a combination of even lines, rhyme, and consonance make for an easy-to-read, almost hypnotic poem. You know how you tend to zone out if you're doing the same thing over and over again, especially when it comes to physical activity? Well, in this case you're lulled into a steady rhythm with all of these evenly-placed sound effects. They sound like the horse as it plods continuously through the field, and the slow and steady sound of the plow turning over the earth with every hypnotic step. Now… snap out of it!