Ironfell is a place entirely antithetical to the Dreaming, having the nature of rigidity and stasis. That is, Ironfell is precisely that place where the accidental attributes of a thing obscure that thing's true nature. Ironfell acts on the Dreaming by petrifying dreamstuff, while the Dreaming acts on Ironfell by doing the reverse.
Ironfell was originally part of the Dreaming, but became separated from it once it was discovered by Eshu.
Once Ironfell was no longer sealed off from the rest of the Dreaming, the places have been unable to exist together. Before the Sundering (which was in large part caused by this separation of Ironfell), when Ironfell was still within the Dreaming, the Dreaming tried to destroy it, but was thwarted by the Sluagh, who gave aid to Nocker, who dwelt there, in exchange for iron knives.
Having split off entirely from the Dreaming, Ironfell became the mortal world.