Got a question about local history? There's a map for that. Or is there…?
This poem really begins with the moment of the title, which acts as a hypothesis or proposition: that the science of cartography (also known as map-making) is limited. The speaker of the poem says that maps don't or can't express everything; their ability to represent the world has its limits.
The speaker then remembers a time when she and her boyfriend were driving around the Irish countryside and they came across a famine road. The boyfriend explained to the speaker the history of famine roads, and how so many Irish died building them. What started out as a romantic road trip took a sharp turn in the other direction; instead of making out, the two ended up discussing the death of millions. Still, it couldn't possibly have been as bad as this date moment.
Anyway, the speaker snaps back to the present and thinks about a map of Ireland, and how amazingly detailed it is. But she knows that this map, like all other maps, is not perfect. For example, it doesn't include traces of the famine roads at all. And since this dreadful part of Ireland's history hasn't made it onto the nation's maps, she reminds herself that she must preserve the memory of this tragedy. Because if she won't, who will? Someone's gotta do the cultural heavy lifting around here, and our speaker proves she's up for the task by writing this poem.