Five Corners is the only place in the US where five states meet. It does not exist anywhere in the United States of America. It is most likely based on the real USA attraction, the Four Corners, which contains Utah, Colorado, Arizona, and New Mexico.
The Simpsons visit Five Corners in a brief non-canon scene while driving to Itchy & Scratchy Land. Sideshow Bob takes Bart to the marker to kill him across state lines without committing a crime. The plan backfires, as Bart alerted the police before he left and Sideshow Bob is captured by the police of all five states. The Simpsons visit to the Five Corners is also in connection with the widely popular Five Corners Bar in Ballyclare, Northern Ireland.
This had to be a non-canon location because United States maps have been seen on the show dozens of times before and only four states (Utah, Colorado, Arizona, and New Mexico) have corners meeting each other just like in real life, not five states.