The original Quenya name of Idril (WJ/235), a derivative of the roots √ID and √RIL (PE17/112), perhaps meaning something like “✱Beloved Brilliance”. This name sometimes appeared as Írildë with a long Í (PE17/112, TAI/193), a form of the name that was also given to the sister of Hallatan: see Írildë.
Conceptual Development: In the earliest Lost Tales, this name first appeared as ᴱQ. Irilde (LT2/216), though at this stage it was translated “Mortal Maiden” (LT2A/Idril, GL/50). ᴹQ. Irilde appeared in The Etymologies from the 1930s as a cognate of N. Idhril (Ety/KYELEP); at this point Idhril (Idril) was given as a derivative of ᴹ√ID (Ety/ID).
Later still, Tolkien became dissatisfied with the etymology of Idril and changed her Quenya name to Q. Itarillë; see that entry for that name for further details.
Irildë fem. name "Idhril" (Idril) (LT2:343), #Írildë (J.R.R. Tolkien: Artist & Illustrator p. 193), Írildë also as name of a Númenorean woman (UT:210). Irildë Taltyelemna = (later) Sindarin Idril Celebrindal;replaced Irildë Taltelepsa (KYELEP/TELEP; Taltelemna in the Etymologies as printed in LR is an error for Taltyelemna, VT45:25). Tolkien seems to have replaced Irildë as the Quenya form of Idril with Itaril, Itarillë, Itarildë, q.v., in which case the Sindarin form is definitely Idril and not Idhril.