What is the significance of the symbols and images in Seamus Heaney's poetry, with examples from his poem "After a Killing"?

Seamus Heaney's "After a Killing" is a poem about the Irish Troubles. Heaney uses symbols and imagery to emphasize not only the struggles of the people, but the inevitability of trouble returning to Ireland. The poem begins with the stark image of two men with rifles, and the suggestion that this image is not new—instead, they were "as if our memory had hatched them, / As if the unquiet founders walked again." This allusion brings to mind the idea of a collective racial memory ("our memory") and summons the image of the founders of Ireland, not at rest ("unquiet") but walking again among the hills. In contrast with the men with rifles, this is incongruous and unsettling.
The landscapes of Ireland are depicted through the use of images little elaborated upon and the use of enumeratio: "rain and scoured light and wind-dried stones / Basalt, blood, water, headstones, leeches." Each of these images summons a host of associations in itself: the first line speaks to the Irish weather, while "blood, water" evokes the Biblical "Blood is thicker than water," and "headstones, leeches" speaks to a history of death and illness.
The "stone house" in the poet's memory once caused the heart to "lift," but the contrast to the girl in the modern day is again unsettling—the food she carries seems plentiful, but "the mould fresh upon them" reminds the reader of the continuing rot and disorder in the state of Ireland, never eradicated.