Ilkorin was a language Tolkien wrote in the earliest versions of his Legendarium.
In this early setting, Ilkorin was Celtic-flavored. When the Noldoli returned to the Great Lands, their language was much influenced by the phonology of Ilkorin and evolved to the amalgamated language known as Noldorin.
In the later writings, the language was replaced by Sindarin and Avarin. In the Etymologies, some names (for example, the rivernames of Ossiriand) are said to be of Ilkorin origin. However, in later notes, some of those names were renamed or given another etymology.
In this same phase, North Sindarin was conceived, and it replaced many of the linguistic features.