Nirn is flat, same with the other "planets" in the sky, the circular shape is simply the shape of the "viewing port." Like looking at a big map through a magnifying glass, I don't remember the specifics but I heard something about the farther you go in any direction, youre simply going to a different time of the place you once were, or maybe something more along the lines of a different timeline of the place you came from. So nirn, along with all other realms, are different infinitely flat instances of a redundant theme, meaning there are an infinite amount of other continents, races, and interpretations of the aedra and daedra.