is simply a variant of the definite article.
Sindarin
ir
conjunction. (?) when
ir
i
neman
adverb. when, (lit.) during what
@@@ based on the theory that ned = “during”
Elements
Word Gloss ned “first, one more; first; during” man “*what, who”
ir
when
?ir (not used in questions but to indicate time, as in ”when I saw you, I was glad”). This is one of several possible interpretations of the word, which occurs in a Sindarin poem untranslated by Tolkien (ir Isil ammen Eruchín…síla, ?”when the Moon shines for us Children of Eru…”, The Lays of Beleriand p. 354). By another interpretation, ir is simply a variant of the definite article.(relative pronoun), see THAT
ir
when
(not used in questions but to indicate time, as in ”when
This word is not translated. It could be related to Quenya íre "when". Some scholars also consider that it could be the form taken by the article i before a vowel, on a pattern similar to ah . To this respect, it might be interesting to note the ir was the allative/dative form of the article in the old Gnomish lexicon, PE/11:9