Half of these questions have Angus spelled as "Aonghais" and the other half have it spelled as "Aonghas", which is correct? Or are they just two different variations?
'Aonghas' is the nominative case, Aonghais is the vocative. In the sentence you referred to, Aonghas is being addressed, so is in the vocative. In theory it should be 'a Aonghais' - 'oh Angus', but the 'a' is lost as it precedes a vowel.
Thank you for FINALLY clearing this up for me! Ive been struggling with this chapter.
There's no 'a' after the "Thu" here, but the same phrase with Beathag did have an 'a'. Why?
Because Aonghais starts with a vowel, meaning the "a" before it is dropped. Beathag starts with a (lenitable) consonant so when we're addressing her directly we'd say "a Bheathag"