What is the central message of the poem "Peter Street" by Peter Sirr?

The main message of this contemporary poem is that it is good to let go of grief over the death of a loved one—and beyond that, it conveys that the imagination has the power to heal and replace sadness with joy.
In the poem, the speaker passes the place where a hospital once stood, the hospital in which his father died. The speaker looks at the site and thinks that he "almost pray[s] some ache remain." In other words, he wishes, for a fleeting moment, that he could feel some grief. The key word, however, is "almost." The speaker doesn't really wish to grieve. As he says, "Forget all that." Instead, he decides to imagine the place of his father's death is a "marvelous house" in which he would hear music and "St. Valentine's stubborn heart"—love—would set his father free to be healthy and whole.
The speaker's father will never be alive again, but the speaker has the imaginative power to retell the story as he wishes and envision him in a positive way.

The central message of "Peter Street" is that although the world changes and people die, the memories of who they were and how they impacted those who knew them don't disappear. Instead, those memories are carried on in the hearts of those who knew the deceased and can be called upon in times of distress. When the narrator walks down the street, he remembers the hospital where his father spent his last days. At first, he misses being able to see his father's face in the window. Because of this, the fact that the hospital has been torn down and turned into a building site saddens him. But as he walks along, he realizes that even if the hospital had remained, it still would not bring back his father. Instead, his father remains in his memory. In his memory, he is able to see more of who his father was and how he made his mark on the world. Because of this, he is at peace with losing the last tangible evidence of the place his father spent his last days. He looks at the building site with hope rather than sadness. He hopes that the new building will bring a vibrance back to the street in much the same way that the memory of his father brings happiness to him. 
https://www.lyrikline.org/en/poems/peter-street-8451

The central message of the poem is that while people die, their memories never do, and it's up to the survivors to decide what to do with those memories. When the narrator passes the site where his father once looked down from his hospital bed, the narrator feels loss when he sees a construction site where the hospital once stood. At first, the narrator wishes for some trace of his father, the father he was loathe to leave and who he stared up at in the window of the hospital. However, in the end, the narrator hopes that the site where the hospital once stood will be turned into a joyful house that will take away the traces of pain that remain from the father's suffering soul. The narrator holds the memory of his father when he walks by the site of the old hospital, but he wishes that the new construction will turn his memory from something sorrowful to something joyful.