In From the Frontier of Writing, since the author's name sounds Irish and the second line of the poem refers to "the troops," I believe the setting is probably Northern Ireland, where there have been many acts of terrorism throughout the years. The troops mentioned in the poem are probably tying to contain the violence between the Catholics who want to be part of Ireland and the protestants who are loyal to England. The first four stanzas of the poem describe the feelings of intimidation and humiliation felt by the author, who is most likely Catholic, as he is scrutinized by the English troops as he passes through a traffic checkpoint. He describes the "tightness and nilness" around him, and describes the soldiers holding guns on him. He also says he feels "interrogated." After he passes through the checkpoint, he feels "emptier, spent, subjugated, obedient." This experience has made him feel like less than a human being.
The first line of the fifth stanza, half way through the poem, makes a transition: "So you drive on to the frontier of writing." Here, now that he is through the checkpoint, he proceeds to try to write about this experience once he gets home in order to deal with his feelings about it. In the final four stanzas, he re-lives the experience and re-tells it in new words. This time, he adds a comment on the feeling of being "freed" and as he came out of the checkpoint. I believe that by giving the poem this title, Heaney is saying that when the writer sits down to write about an experience, he is on a frontier - a new territory where his feelings about the experience will take him. He will discover how the experience has affected him and what meaning he will take from the experience while he is on the "frontier," thinking and writing about it.