Ade wrote:
I should note, also, that this sound rule is only true of uninterrupted speech. If you were to say the words slowly, one after another, as you might to a child who is learning to speak, you would absolutely pronounce both vowels separately, /u:Nə ətɑ: orəm/. It's the same as in the English phrase "The sofa at work". You would run the final a of "sofa" into the first a of "at" if speaking quickly, but if pronouncing the words slowly, as you might to somebody who is not very proficient at speaking English and finds it difficult to understand quick, fluent speech, you'd deliberately pause between each word and therefore pronounce the vowel twice.
Well, in Southern England, we have the "intrusive R". "The sofa at work" is pronounced "the sofar at work". I believe they do things differently in the US, and probably in Ireland. In Southern England /ə/ cannot run into /ə/ without a glide consonant /ɹ/. The exact glide consonant (semi-vowel or liquid) used to separate vowels in English (Southern English English) depends on the vowels. In the environment of /ə/, it is /ɹ/. But in "I am", the pronunciation becomes "I yam", with /j/ being the glide consonant. In "to understand", the pronunciation becomes "to wunderstand" with /w/ being the glide consonant. So, as you can see, it is not incorrect to produce such glides, and the intrusive R is not "wrong". Handling adjacent vowels is a feature of a language's phonology. In America, they say "to understand" going from /tə/ into "understand" with no glide consonant, as the dialectal phonological rules are different State-side.
But in Irish, vowels definitely are elided before other vowels. The vocative "A Eóin" is pronounced just "Eóin". As you say, Ade, speed of delivery is important here. If you're speaking disjunctively or particularly emphatically or slowly, then it would be "A Eóin", as /ʔə ʔo:ᵊnʲ/, where /ʔ/ is the glottal stop. In English, a glottal stop is produced before a vowel where the glottis is not already open. We don't normally notice this as a consonant, but it is one, and is written as such in Arabic (and the /ʔ/ symbol is taken from the Arabic alphabet).
Geoff Lindsey, the UK linguist, has commented on "hard attack", which is spreading in English. Hard attack is the German way of producing a glottal stop before every word beginning with a vowel, even if it is not the first word in a sentence. E.g. "um elf" in German sounds distinctive because both words start with a glottal stop, giving German its staccato sound, whereas in English "an elf" (which doesn't mean the same thing) traditionally only has a glottal stop before the "an" and not the "elf", because the vocal cords are already open to produce the next word, even if beginning with a vowel. Lindsey points out that more and more young English people (and Americans) are beginning to pronounce all words starting with vowels separately in the German style. I think this is also why more and more people are saying "a apple", and not "an apple", because if you use hard attack, "apple" begins with a glottal stop, and so needs no "n" to separate it from the article.
Learners pronounce phrases like "A Eóin" effectively with hard attack, although this wasn't the traditional way.
See
https://www.youtube.com/watch?v=KFZZI7HCp2M for Lindsey's video on Hard attack.