You can relax a bit. Heaney isn't beating around the bush with this title. It's fairly straightforward. He lends us a helping hand by giving us a sneak peek of the action in the poem (a storm) and the location (an island). It's not too much more specific than that though—we don't know what kind of storm (for all we know it could be a thunderstorm, a hurricane, a snowstorm, or tsunami). And we don't know what kind of island we're dealing with either—we could be in Hawaii, Madagascar, or Manhattan. Sure, we can guess that Seamus Heaney, an Irishman, means Ireland. But then why not call the poem "Storm on Ireland"?
Instead, we're left with two broad concepts: "storm" and "island." In that way, the title invites us to consider the general threat posed by nature (in the more generic form of a "storm"). Importantly, it also invites us to consider our own isolation as human beings when faced with such a threat. We're on an island, if not literally, then metaphorically. In other words, we're isolated, left alone to face the unrelenting force of our environment. Better zip up your slickers, gang.