quote:I learned in LI that the present tense of the copula in certain forms added a 'b' in front of the third person pronouns.
It is a local phenomenon in some dialects, .i. it is not universal.
quote:"Ab úlla é seo" or "Ar úlla é seo"?
Your question is a misconception. Even if a dialect uses "ab" in these situations it is copula of
present tense. "ar" is a past tense copula. The corresponding form of past copula in front of vowel is "arbh", not "ab"!