"Punishment" is based on an actual event in 1951 when the body of a young woman was found in a bog in Windeby, Germany. The girl lived in the first century A.D. According to the Roman scholar Tacitus, Germanic people punished women accused of adultery by shaving their heads, sometimes torturing them, and sometimes killing them.
In the poem, the poet imagines the moments before her death, the tug at the collar around her neck and the wind shaking her body. The poet then comes back to the present and imagines witnessing her being dug up. He considers how beautiful she must have been nearly 2,000 years ago. Then the poet says, had he been there, he would have stood by, said and done nothing.
I almost love you
but would have cast, I know,
the stones of silence. (29-31)
This references the Gospel of John, 7:53-8:11, where Jesus does step in and prevents a woman from being stoned to death after she'd been charged with adultery. Aside from those like Jesus, this indicates that at any time in history, people are most likely to comply with current practices (including punishments). The poet supports this by including a reference to how this young woman's "sisters" were cauled in tar and wept by the railings. This reference is about young Catholic women in Northern Ireland who were tarred in feathered as late as the 1970s for having relationships with British soldiers. These sisters would be outraged at the young girl's punishment but would sadly understand that each era has its values, fanatics, and common but cruel practices.
The poem deals with harsh persecution of women, so this poem does have a feminist criticism of men's historical abuse of women. But the poem is also both an indictment and an acknowledgment of those who stand by while others suffer. It raises questions such as, "had you been alive in the 1850s in America, would you have been at the forefront of the abolition movement?" In any time in history, had you been alive, would you simply abide by the practices of the day, no matter how cruel they might be?
Heaney uses a lot of enjambment in this poem, which just means a line that continues onto the next line. It is a broken, stilted style in which the reader might pause as the line continues to the next. The brokenness could apply to the broken body of the girl. The pauses cause the reader to stop, consider the images, and consider what he/she might have done at such periods in history, had he/she been there to witness this punishment.