The poem's imagery helps readers to feel the speaker's fascination with the Tollund Man, and his horror over the Troubles.
The poem begins with a portrait of Tollund Man. Preserved in a bog for millennia, he seems almost to have become part of the earth: his skin is the same "peat-brown" as the earth around him, and his eyelids have become "mild pods," like the ancient plant matter that makes up peat. His "pointed skin cap" feels earthy, too—and also marks him out as someone from a distant, long-vanished culture.
This imagery paints the Tollund Man, not just as an earthy figure, but as a gentle one. Anyone who's seen a picture of the Tollund Man will know that he looks as if he were sleeping; the speaker's attention to his "mild" eyelids captures that peaceful feeling.
The poem's next images, however, paint a more forensic picture. The speaker knows that the Tollund Man's "last gruel" was "caked in his stomach," preserved (and compressed) just like his flesh. But of course, one can't cut open a sleeping person's stomach to find out what they ate. This image underlines Tollund Man's emphatic deadness.
The Tollund Man isn't just a peaceful sleeper or a crime scene body, however: he's also a religious sacrifice. When the speaker imagines how the "goddess" allowed the "dark juices" of the bog to slowly stew the Tollund Man, his imagery suggests that those "dark juices" might be, not just literally dark-colored, but full of sinister magic.
After this portrait of the Tollund Man as a calm sleeper, a crime victim, and a sacrifice, the speaker's pictures of the Irish dead feel a lot simpler and grimmer. Like the Tollund Man, some of these figures are marked by their clothing: they're "stockinged corpses," looking vulnerable in their socks as they lie dead in their own "farmyards." Some, though, have been reduced to "tell-tale skin and teeth / Flecking the sleepers"—mere stains and scraps, speckling the railroad upon which they were dragged to death.
The poem's imagery thus helps readers to imagine all these different (and similar) dead bodies: their shared humanity and their shared suffering.