Of course they were based, to at least some considerable degree, on Earth races/nations/cultures. However, contrary to the OP's misapprehension, there is no one-to-one correspondence between any Nirnian race/ culture and any Earth one. Instead, the Nirnian races borrowed bits and pieces from many Earth races and cultures and cobbled them together with some measure of pure fiction and ended up with what they ended up with.
But you can still distill where those disparate elements came from. Such as Bosmers obviously Amazonian jungle dwelling cannibalism, and their Wild Hunt, which is obviously Celtic.
Are the beast races and dark elves inspired by anything?
The Dark Elves definitely take a lot of cues from both pre-Islamic and post Islamic middle east. Hlaluu architecture is like that of Babylon or Sumeria, whereas the Ashlanders are much like the nomadic Beduoin, Tuareg, etc.
There are also other elements (some of the clothes were Native American inspired, the religious system of the Ashlanders is more matriarchal than any Middle Eastern religion I'm aware of)
There was some guy on the first page who cleverly explained the Kahjiits, the Argonians I would say are a little like Papa New Guinians, basically tropical tribesman with a lot of exposure to water (also, Trees were sacred to them as they were in limited supply on island habitats, could provide food (the pith, grubs) and were made into buildings and canoes.
There are probably more correlations I can't make. And most people wouldn't. Which is why they are convinced that ES races are complete fabrications which can never be compared to human cultures. Seriously, we could get a new race called the New Yorkers and they could ride subways and be rude to you and eat bagels and people would say that " TES New Yorkers =/= New York New Yorkers"