Hate to really get fiddly with the details...
... BUT I'm pretty sure we call our planet "Earth" because it is made of "earth".
And so it sorta stands to reason that the dirt we refer to as "earth" probably wouldn't be called that either on Nirn.
In fact I'd go so far as to say the dirt should probably be called "nirn"... much as the planet itself is "Nirn".
=D
Afterall... this might sound damned stupid to you.... but it is the nature of sentient life to only begin naming things when there is more than one of them. As such, the first of them always tends to carry a generic term referencing what it is to the one who named it. This is why I always presume that in any fantasy or sci-fi... country / planet names, IF named thus by their inhabitants... probably just mean "home" or "the place where stuff happens" or something like that in their native language.
Nirn refers to the planet where the continent of Tamriel is on.
Nirn is Ehlnofex and means arena. Tamriel means dawn's beauty.
When you refer to Nirn, and its orbiting moons and planets, you talk of the Mundus.
The Mundus floats in Oblivion, where the Daedric princes also have constructed their realms.
Beyond Oblivion lies Aetherius where Magnus and the Magna-Ge went to when they retreated from creation.
All of this, together with outer realms not mentioned above is called the Aurbis.