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 “that is the reason why” using the relative pronoun ya more commonly used in declined forms in later writings, such as Q. yassë “where [relative]”. For example: istan i saila atan yaro i osto alya (ná) “I know the wise man that is the reason why the city is prosperous”.