Alliteration is first used in the opening two lines. Here, the speaker discusses the kind of dwellings that the islanders have made on their land:
We are prepared: we build our houses squat,
Sink walls in rock and roof them with good slate.
These /s/ and /r/ sounds draw attention to the poem's construction, in turn suggesting the building efforts of the islanders. That is, by making the reader more aware of the poetic line as something that is made, the poem conveys a sense of effort and method in keeping with the discussion of building sturdy houses. Interestingly, though, the /s/ sound (which is also known as sibilance) in these lines is airy, reminiscent of the wind that howls across the island. Accordingly, the alliteration simultaneously speaks to the quality of the houses and to nature's ability to potentially destroy them. The /s/ alliteration in lines 4 and 5 supports this idea further.
The next significant example of alliteration is in lines 6 and 7:
Which might prove company when it blows full
Blast: you know what I mean - leaves and branches
These /b/ sounds are the equivalent of the poem turning up its volume, happening just as the speaker discusses the loud noise of a storm blowing through trees (and possibly alludes to explosive devices). The two /c/ sounds in line 8 are similarly loud and hard, as are the two /f/ sounds of "fear" and "forgetting" in 9 and 10.
In line 11, the three /n/ sounds emphasize the poem's focus on negative space—on the lack of something rather than its presence (the threat of the storm as opposed to an actual storm). They place weight on the word "no," anticipating the "huge nothing" in the last line.
In lines 14-16, the poem likens the coming storm (which may be an extended metaphor for violence in Northern Ireland) to a "tame cat / Turned savage." These lines also go in heavy on hard /t/ and harsh /s/ sounds, as well as a repeated /b/ that recalls the "Blast" of line 7:
But no: when it begins, the flung spray hits
The very windows, spits like a tame cat
Turned savage. We just sit tight while wind dives
This alliteration combines with internal consonance based on the same sounds to give the lines a spitting, menacing quality. Line 17 picks this up, with three words that also start with /s/.