I knew about the French. But since Irish is not a Romance language, I'm constantly surprised by the Irish words that actually are cognate (as opposed to more recent borrowings). So what struck me was how much ubh sounds like œuf, and the fact that it's not a coincidence. Since, as you said last month, ubh is cognate to ovum but not descended from it, that suggests to me that the parent word probably traces back to Proto-Indo-European.
(I've dabbled in a number of languages, but Irish is the first Indo-European language I've studied that isn't a Romance language. So I'm not really sure what to expect on the scale from similar to different.)
While Irish spelling is fairly regular, from a phonetic point of view (and taking dialect differences into account), it's not rigid, and older, simple words are often the ones that differ most. ubh has a "v" sound in all regions, but dubh only has a "v" sound in Munster.
The key thing to remember is that written orthographies are only guidelines.
When what you read conflicts with what you hear, assume that what you read is wrong. Written descriptions never describe all the exceptions fully, and can only be taken as approximate guidelines.
In this case, "ubh" has an "v" sound in all dialects, whereas "dubh" only has a "v" sound in Munster Irish, and a "w" sound in Ulster Irish, and is just "du", with neither a "w" or "v" sound, in Connacht Irish (at least some speakers of Connacht Irish - the speaker on Duolingo pronounces "dubh" with a "v" sound).
And while the Caighdeán has no formal standard pronunciation, most "non-dialect" speakers pronounce "dubh" with a "v" sound.
In Connemara, generally, it depends on where in the word the sound is. At the end of the word, or before a consonant, it is sometimes pronounced as /v/. Unless, of course, the unlenited form would've needed an emphentic vowel, in which case it is /u:/. Such is the case in garbh, marbh, etc, where it would be said "garabh", but became "garú". As for dubh, it's more just the final sound is occasionally elided, but it doesn't elongate the vowel like it does in Ulster. So you get something that sounds like "du" instead of "dú", which is actually the /w/ following it in Ulster.
So, really, it depends on dialect and position in the word, as well as what vowels and such proceed it. It's not that what you learned was wrong, just that it doesn't tell the whole story.
Even today the vast majority of the time someone says eggs without further qualification what they're referring to is uibhe circe. Furthermore even if the variety of eggs available to consumers has vastly proliferated since his day I don't see how his point is undercut at all. He notes that in his day, native speakers tended to explicitly state which kind of egg was being discussed e.g. ubh circe or ubh lachan etc. to the point that it was unusal to here ubh alone. The only thing having Cadbury creme eggs around should have changed is now ubh uachtair is added to the lexicon.
I don't mean to suggest that there's anything wrong with what's here in this exercise. Egg is certainly the correct translation of ubh. I just felt like since one of the most important speakers of modern Irish explicitly commented on this word it would be worth mentioning his thoughts here.
Even today? The variety of eggs available to consumers today has not vastly proliferated since his day - in his day a grocer would by just as likely to sell a goose egg or a duck egg if their supplier turned up at their door with them, and it wasn't unusual for people living on the islands to gather gulls eggs, so specifying the type of egg made sense at the time.
Nowadays, when most Irish speakers have never seen, never mind eaten, a goose, duck or wild bird egg, it's silly to suggest that you should write ubh circe on your shopping list.
While "creme" and "cream" might be homonyms in English, I don't think ubh uachtair is likely to get much traction either.
You wrote "An tAthair never set foot... where you can choose between a dozen brown eggs in various sizes or a Cadbury Creme egg." This led me to believe that you thought it was the dazzling variety of oviform products available in these stores that would overwhelm An tAthair's faculties in the Irish language. Pardon me if I misinterpreted the nature of your contention.
Either way the fact of the matter is that to the old monoglots, the eggs in our supermarkets are uibhe circe (or uibheacha circe outside Munster). Following their example, as I think is wise for any learner, I believe we should also call these things uibhe circe. The fact that as Béarla they're just eggs and not hen-eggs is irrelevant, a red herring. The suggestion I find silly is that we should fix the shape of an Irish word or phrase by direct recourse to the nearest English equivalent.
And I agree that ubh uachtair won't be catching on any time soon, though uachtar is given in the EID translation of creme de menthe.
I did a bit of research, and "ubh" is not a borrowed word at all. It is cognate with the French "oeuf" because both words trace back to the same Proto-Indo-European word. The Celtic languages are every bit as European as the Romance and Germanic languages.
Broad bh is "v" in Munster Irish. Ubh is pronounced with a "v" sound in all dialects of Irish. Ulster Irish speaker might prefer to spell it ibh, but ubh is the widely accepted spelling.
While the pronunciation of Irish generally is more faithfully reflected in the spelling than English pronunciation, never assume that an otherwise competent speaker is doing it wrong because their pronunciation doesn't match your interpretation of a written guide to pronunciation. Apart from the fact we have a fairly standardized spelling in spite of significant dialect variation, certain words, particularly the kind of words that children learn to say before they learn to read, can have distinct pronunciations that will not conform to spelling norms, because it's the spelling that is the problem, not the pronunciation.