There are quite a few attested plurals in The Etymologies for both Doriathrin and Ilkorin nouns. From these we can deduce a number of features for the Ilkorin and Doriathrin plurals.

The plural suffix -in: With two exceptions (see below), all the attested plurals in both Doriathrin and Ilkorin use the suffix -in. This seems to be the basic mechanism of plural formation for nouns. It is likely this suffix was inspired by the Gnomish plural suffix -in¹.

The i-affection of [a] and [o]: The plurals of Ilkorin nouns containing short vowels [a] and [o] have these vowels shift to [e] and [u] according to the normal rules of Ilkorin i-affection. The clearest examples of this change are Ilk. lalm → pl. lelmin (Ety/ÁLAM) and Ilk. thorn → pl. thurnin (Ety/THOR). This change did not occur for long vowels, such as Ilk. tôr → pl. tórin (Ety/BAL); long [ā] did not normally occur in Ilkorin.

This i-affection does not seem to occur for plurals in the Doriathrin dialect: Dor. roth → pl. rodhin (Ety/ROD) and Dor. than → gen. pl. thanion (LR/405). In fact, there is one noun orth that has plurals in both Ilkorin and Doriathrin, with Ilk. pl. urthin (EtyAC/WATH) versus Dor. pl. orthin (Ety/ÓROT). This lack of i-affection seems only to be true of the noun plural and not the general phonological history of Doriathrin: consider Dor. Denithor and Dor. Thuringwethil, in which an older [a] became [e] before an [i].

Syncope of vowels in final syllables: In polysyllabic Ilkorin nouns, the vowel in the final syllable is lost when the plural suffix is added: Ilk. adar → pl. edrin (Ety/ATA) and Ilk. talum → pl. telmin (Ety/TAL). The i-affection of [a] and [o] noted above also applied in these cases. This change did not occur when the result would be an invalid cluster of vowels: Ilk. Balthor → pl. Balthorin (Ety/BAL), but could occur where the cluster was acceptable: Ilk. gangel → pl. genglin (Ety/ÑGAN).

Note that there are no attested examples of plurals for polysyllabic Doriathrin nouns, so it is unclear whether this rule would apply to that dialect. However, based on the syncope in the genitive inflection Dor. naugol → naugla (Ety/NAUK), it seems likely that this rule would apply to Doriathrin as well.

Preservation of stem forms: In some nouns, older stem forms were preserved in the plural. For example, in Doriathrin a final [ð] became [θ], such as ᴹ✶roda > [roð >] Dor. roth (Ety/ROD, EtyAC/ROD). The older form [roð] is preserved in its plural form rodhin. Similarly, the word Dor. regorn “holly-tree” still uses its simpler form reg “holly” in its plurals: Dor. pl. regin and gen. pl. region (Ety/ERÉK).

Plural using only i-affection: There is one Ilkorin plural produced exclusively by i-affection without the suffix -in: Ilk. tal → pl. tel (Ety/TAL). Without further examples, however, it is unclear when this variant plural would or would not occur.