"Punishment" is full of precise and detailed imagery, which brings to life not only the appearance but the backstory of the executed "adulteress."
The speaker, writing in modern times, tries to empathize with her long-ago experiences: he claims that "I can feel" and "see" what happened to her. Using tactile (touch-based) imagery, he imagines "the tug / of the halter," or noose, on her "neck," and "the wind" on her bare body in her last moments of life. Using visual imagery, he also imagines things she couldn't have seen, including "her drowned / body in the bog" and "the floating rods and boughs" around her.
After recreating this scene, he describes the appearance of her body as it was dug up from the bog. This account seems based on photographs of a corpse formerly called "the Windeby Girl," discovered near the German town of Windeby and described in Heaney's source material, the 1965 study The Bog People. (The corpse later turned out to be male.) He points to visual details mentioned in that book, including the "shaved head" and "blindfold" that seem to indicate death by execution. He also sketches an anatomical portrait of the corpse's "brain[]," "muscles[]," and "bones." The perspective here feels unsettlingly intimate, or perhaps tenderly human, rather than clinical, as the poet labels himself an "artful voyeur." Likewise, the speaker affectionately imagines how the girl appeared in life: "flaxen-haired, / undernourished," yet "beautiful."
The poem's final imagery involves punished women in Seamus Heaney's own day: women who were tarred and feathered for their alleged "betray[al]" of the Irish nationalist cause. The reference to women "we[eping] by the railings" they were tied to offers a brief, yet powerful visual portrait. The image "cauled in tar" is partly tactile, too, as it prompts the reader to imagine hot tar sticking to the skull like a "caul[]," or membrane.