"I did not make an exception."
Translation:Ní dhearna mé eisceacht.
Níl is the negative form of tá - you don't use níl with other verbs. As tá is the present tense form, níl is only used in the present tense.
déan is an irregular verb, and it's irregular past tense forms are mentioned in Verbs 1: Past notes - https://www.duolingo.com/skill/ga/Verbs%3A-Past/tips-and-notes
One feature of the irregular past tense of déan is that it doesn't use the expected past tense negative particle, which is níor. 6 of the 11 irregular verbs, including déan, use ní in the past tense.
I'm assuming you mean you hear an /a/ from the "ea" portion of the word?
Take this with a grain of salt, because my pronunciation in Irish is absolutely awful.
I think the pronunciation of "each" in the second syllable of a word as either /əx/ or something closer to /ax/ is based on either the situation combined with the dialect or the individual speaker.
Depending on the dialect, /x/ affects stress and pronunciation following an /a/, even with words containing "each", not just "ach". However, in words where the first syllable has a long vowel or diphthong, the sound is usually reduced to the schwa like normal, (see pronunciations for the word éisteacht or eirleach, although I've heard varying things for éisteacht).
My best guess is that, in this word, because the ei here is pronounced like the short vowel e (/ɛ/) (unlike the /eː/ in eirleach), this still applies, as this pronunciation of eisceacht seems to be relatively common, with the emphasis even being on the second syllable sometimes.
Anyways, you can hear some similar pronunciations here, depending on the dialect:
https://www.teanglann.ie/en/fuaim/eisceacht https://www.teanglann.ie/en/fuaim/isteach https://www.teanglann.ie/en/fuaim/misteach https://www.teanglann.ie/en/fuaim/ailseach