It depends. While Belfast is inspired by real-life events and emotions, it's not a strict retelling of a specific true story. Some elements might be fictionalized for dramatic effect.
There's no way to tell who the author of 'a belfast story' is without further details. It could be a local writer from Belfast who wanted to share their insights about the city, or it could be someone from elsewhere who was inspired by Belfast's allure.
Yes, the 'Belfast' film is based on a true story. It draws from the director Kenneth Branagh's own childhood experiences growing up in Belfast during the Troubles.