idir

• eidir

From Old Irish eter, from Proto-Indo-European *h₁enter (“between”). Cognate with Latin inter (“between”) and Sanskrit अन्तर् (antár, “between, within, into”). Compare Scottish Gaelic eadar.

• IPA^(key): /ˈɪdʲəɾʲ/
• (Cois Fharraige) IPA^(key): /ˈɛdʲəɾʲ/ (also spelled eidir)

idir (plus nominative, triggers no mutation, triggers lenition in the meaning ‘both (...and ...)’)

1. between

   idir fir agus mná ― between men and women

2. both (paired with agus (“and”))

   idir fhir agus mhná ― both men and women

In modern usage, this preposition has inflected forms only in the plural. Idir is used with nouns and with singular pronouns in combination with another noun or pronoun:

idir é agus mé ― between him and me

In the meaning ‘between’, this preposition does not trigger any initial mutation. In the meaning ‘both (... and ...)’ it triggers lenition of both the word following it and the word following agus.

From Old Irish etir.

idir

1. at all
   • Chan 'eil idir. ― Not at all.
   • Cha'n e idir. ― That is not it at all.
   • Cha tig e idir. ― He will not come at all.
   • No idir air a bhruicheadh. ― Nor even boiled.
   • Cha'n 'eil e gu math idir. ― He is not at all well.
   • Cha deanainn idir e. ― I would not do it on any account.
2. yet

• Faclair Gàidhlig Dwelly Air Loidhne, Dwelly, Edward (1911), Faclair Gàidhlig gu Beurla le Dealbhan/The Illustrated [Scottish] Gaelic-English Dictionary (10th ed.), Edinburgh: Birlinn Limited, →ISBN
• A Pronouncing and Etymological Dictionary of the Gaelic Language (John Grant, Edinburgh, 1925, Compiled by Malcolm MacLennan)