The correlative ᴹQ. iro appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/111), a combination of the relative pronoun ᴹQ. i and ᴹQ. -ro² “reason”.

Neo-Quenya: For purposes of Neo-Quenya I would update this to ᴺQ. yaro “for the same reason as” using the relative pronoun ya more commonly used in declined forms in later writings, such as Q. yassë “where [relative]”. For example: nilin Quenya yaro lye “I love Quenya for the same reason as you”. Hat-tip to Vyacheslav Stepanov for suggesting this gloss; compare ᴹQ. inárea “as old as” and Q. yallë “in the same way as”.