The poem opens with the words "I can feel," but it soon becomes clear that the speaker isn't talking about his own experience. Instead, he's imagining someone else's. The unidentified speaker—who, later details will suggest, might be read as Heaney himself—is re-creating the experience of a woman who once stood "naked" with a rope around her "neck." The poem's title signals that her hanging was not a suicide; it was an execution, or "Punishment."
The speaker imagines "the tug / of the halter," or noose, as it tightened at the "nape" (back) of the woman's neck; this is a description of the moments just before her hanging, or perhaps of the hanging itself. The imagery is vivid and tactile (i.e., based on the sense of touch), helping readers, too, viscerally imagine themselves in this grim situation.
Normally, the word "halter" refers to a rope used for leading or tying up animals. Here, it suggests that the woman's executioners are dehumanizing her, treating her no better than an animal. Clearly, she isn't "naked" by choice in the "wind[y]" weather; her captors have stripped her in order to humiliate her. As later lines reveal, she has been convicted of a perceived sexual offense—adultery—so her punishment is designed first to shame and then to kill.
Though some religions regard adultery as a sin, few modern societies treat it as a crime, and even fewer treat it as a capital crime (one punishable by death). In modern Europe, where Heaney was writing, public shaming and execution for adultery are associated with more primitive eras; they haven't been part of formal legal systems for centuries. Right away, then, the method of "Punishment" is a clue that the woman in the poem lived long ago. However, the poem will question whether European cultures—and "modern" or "developed" societies in general—have truly progressed beyond such cruelty.