The poem opens with a small but important word: "we." This first-person plural pronoun signals immediately to the reader that this is a poem about community and togetherness; coupled with the slightly menacing title, already there is the sense that this will be a poem that examines the relationship—and sometimes conflict—between humankind and nature.
The caesura in the first line indicates that the speaker intends to explain just how the islanders are "prepared" for a storm. They build houses "squat"—which means these dwellings do not go very high—which helps protect them from potential damage from high winds. The consonance and alliteration (which is itself a form of consonance) across this line and line 2 reinforce that idea of a solid structure:
We are prepared: we build our houses squat,
Sink walls in rock and roof them with good slate.
The selection of sounds here feels deliberate and methodical, gently suggesting the building of "squat" houses. That said though, the numerous /s/ sounds (also called sibilance) have a windy quality to them as well, which suggests the threat of a potential storm.
Lines 3 to 5 offer the reader more information about the island in question. The earth upon which the islanders have built their homes is "wizened," which means shriveled with old age. This makes it sound like a barren, near-inhospitable environment—which is confirmed by the fact that this same earth "has never troubled" the islanders with "hay." The speaker is being ironic here—hay would be useful to them, but it seems that the island doesn't provide anything useful.
The mention of trouble also foregrounds the way that, at some point, the islanders are expecting trouble. It's worth noting here that this specific word—"trouble"—can be taken as a subtle hint to the reader that the poem is an extended metaphor or allegory about the tensions in Northern Ireland. Though the poem was published near the beginning of the 20th-century period known as the Troubles, the name had been used to describe earlier Irish conflicts too. The Troubles is best known as the term that describes ongoing violence and tension in Northern Ireland (Heaney's home nation) during the late 20th century between, put simplistically, those who wanted to unify with the rest of Ireland (the Republic) and those who preferred to stay part of the United Kingdom.