'island' is homophone for Ireland. the poem is an extended metaphor for the political storm that raged across northernireland in late 1960s.
'we are prepared' shows how they are part of a community and this is a universal experience. also creates a sense of foreshadowing as this simple statement sets the tone for the rest of the poem.
'we build our houses squat' shows the houses are sturdy and secure, but also shows how this suffering has been going on for a longtime, they have planned for this.