Interestingly enough Plato’s idea of banishing the poets from his imagined society has worked its way into the history of the world and its civilizations. Marx himself dismissed art from his communal society, since it distracts the individual from the progression of the whole. In a more applied sense, Hitler destroyed thousands of pieces of art that he found disagreeable with his world view and how he wanted to rule his expanding nation.
This sets me up for my answer to your question. I think a society has both the ability to benefit or suffer from artistic depiction found in poetry, paintings, etc. You have provided one example already of a society benefiting from art depicting violence. Goya’s Third of May, paints a picture of the cruelty and gruesomeness of war. If you look at a photo of the painting you will see a figure dressed in white, his arms raised causing his body to represent a cross, about to be fired upon. Goya transforms this victim of a firing squad into an artistic symbol. The man is innocent, pure, and a martyr for the Spanish cause. This piece of art because of its symbolism and its bias will sway the viewer to categorize the victim as the good guy or at least pity him, therefore, potentially swaying their opinion of the war to align with the cause of the Spanish. Here is evidence of art as propaganda.
I think both Yeats and Heaney’s poetry had an effect on the violence that was prominent during their lifetimes. I think both of them furthered this violence whether that was their intention or not. I will strictly talk about Heaney since that is what you asked about. In North, Heaney creates this idea of tribes and tribal rites in most of his poems. I want to look at “Funeral Rites” specifically. At the very beginning of the poem Heaney writes, “I shouldered a kind of manhood / stepping in to lift the coffins / of dead relations.” There is a ritual here, one of ascending into manhood, becoming a member of the tribe with this evolution. Later in the poem Heaney gives meaning to this ritual and also to the murder of this man, writing: “I would restore / the great chambers of Boyne, / prepare a sepulchre / under the cupmarked stones.” The Boyne is historically where the kings of Ireland were buried. This ceremony gives meaning to the murder, celebrating the deceased as a martyr whether Heaney intended it or not. This could potentially further the violence already taking place while the poem is being written. This could be viewed as art being used for the furthering of negative acts. In both instances the art is being received and interpreted by the readers and is a form of propaganda.
So to answer the question, I think Heaney’s work is somewhat problematic because he focuses his work on death, but in such a way that makes it almost glorious and purposeful to die. Since art is propaganda there have ben many tries to control it throughout history. In a perfect society where change is not necessary and no conflict arises then art would not exist since it would have no purpose, no meaning. Like Yeats believed, the creative process comes from conflict within oneself or from the outside world.