The Gnomish Lexicon of the 1910s had G. erw “only” as an adverbial form of G. er “one” (GL/33). A similar adverb ero appeared in the Túrin Wrapper from the late 1950s or early 1950s in the phrase ach in ellath hí ero ed epholar (VT50/5), untranslated but probably meaning something like “but now the Elves alone (?remember) it” as suggested by Carl Hostetter (VT50/15). The first version of this phrase had eno instead [perhaps meaning “*still”], and the third version had îr.

There was another note probably from the late 1950s that derived îr from adjectival ✶eryā, but this phonetic development is abnormal since it seems to assume i-affection occurred after the loss of final vowels rather than before: eryā > erı̯a > erı̯ > irı̯ > îr (VT50/18). See also S. air “lonely” from the early 1960s which has the expected phonetic development from primitive ✶eryā.

Neo-Sindarin: For purposes of Neo-Sindarin, I would use ero “only, alone” as the best of the available options for the adverbial forms. For the adjective I would use S. air “lonely”.