Ilmarin referred to the mansions of Manwë and Varda.
Other uses
In one poem, the name Ilmarin is used for Valinor, and in the Eärendillinwë the name "Hill of Ilmarin" refers to Oiolossë.
Description
The domed halls situated on the summit of Taniquetil, the highest peak of the world, from where Manwë here set his throne, and spirits shaped like hawks and eagles constantly came with news of events in Arda. Manwë and Varda "could look out across the Earth even into the East".
When Manwë there ascends his throne and looks forth, if Varda is beside him, he sees further than all other eyes, through mist, and through darkness, and over the leagues of the sea. And if Manwë is with her, Varda hears more clearly than all other ears the sound of voices that cry from east to west, from the hills and the valleys, and from the dark places that Melkor has made upon Earth.
During the high feast before the Darkening of Valinor, the Maiar, Vanyar, and Noldor sang before Manwë and Varda in their halls.