It's not the first ever Beatles song. In fact, when it was first released it was credited to "Tony Sheridan and the Beat Brothers." So the first Beatles song would have been later. Unless you count the Quarry Men stuff like "In Spite of All the Danger" as Beatles. But even then, "My Bonnie" wouldn't be the first-ever Beatles song.
It's not the first ever Beatles song. In fact, when it was first released it was credited to "Tony Sheridan and the Beat Brothers." So the first Beatles song would have been later. Unless you count the Quarry Men stuff like "In Spite of All the Danger" as Beatles. But even then, "My Bonnie" wouldn't be the first-ever Beatles song.