I'd regard this sentence with extreme suspicion, it doesn't seem correct in any case. As far as I know the primary use of an ea is as a tag question coming after a whole clause e.g "An capall a dhíol a rinne sé, an ea?" = "He sold the horse, right?" or Fanaimís inár dtost, an ea? = "We should stay silent, right?" A secondary case is preceding a direct relative, where it means something like "Is it that..." or "is the fact of the matter that..." e.g. An ea atá sé i bhfeirg liom?= "Is it that he's angry with me?" Neither of these two usages is represented here.
Ea doesn't occur in normal copular questions. If I wanted to ask "Is it a donkey or a horse?" I'd say An asal nó capall é? If I wanted to ask "Was it Paul or Patrick whom you saw?" I'd say An é Pól nó Pádraig a chonaicís? There's no ea in either of these questions. By this same logic there should be no ea here if it's to be a copular question.
I also have doubts about the suitability of the copula here at all, but this is getting long and I'm less certain of those so I won't go into them. Even if it can be used the grammar displayed here almost certainly isn't correct.
The acceptable uses of ea can be found here. To use sé or sí, the question would need to begin with An bhfuil sé … ? or An bhfuil sí … ?. My guess is that in this case, it’s really an elision of something like An ea [go bhfuil an méid] ocht nó ochtó? (“Is it [that the amount is] eight or eighty?”).
I saw it as a reasonable, if less literal, translation of the proposed elided form above — An ea [go bhfuil an méid] ocht nó ochtó? (“Is it [that the amount is] eight or eighty?”), with “Are there eight or eighty?” also expressing an inquiry about the amount. If I were translating from English to Irish, though, I’d probably use the translation that you’d offered.
I'd say An é go bhfuil..?, but I think my problem is going from the singular to the plural.
Having said that, I note that the FGB does have the example of Deartháireacha iad, nach ea? - "They are brothers, are they not?"
But I think it's a bit of a stretch to rely on an elision to justify an ea, and then say that it's reasonable to translate to an English phrase that can't be used with the elision (you can't say "are there [that the amount is] 8 or 80".
Since I’m not fluent in Irish, I try to make sense of the exercises here as presented; sometimes the exercises here are wrong, and sometimes the exercises show structures that I was previously unfamiliar with (e.g. the use of a predicative adjective in Tá an t-amharc go dona aici, since its English translation uses an attributive adjective; an English translation would need a relative clause to also be able to use a predicative adjective, despite the Irish sentence not having such a clause). Translations aren’t always straightforward, and I don’t doubt that some stretching could be needed.
Clearly there are many translations that require some stretching, because the structures used just aren't the same in both languages. I don't have any problem with that.
But unless "is it that the amount is 8 or 80?" means exactly the same as "are there 8 or 80?", then it may be misleading to suggest that "are there 8 or 80" is a good translation, particularly if it loses the nuance between the copular construction and the Bí construction.
That was my point above; I’d noted that the elision was a guess on my part, starting from a presumption that the Irish sentence is correct. If the An ea … sentence is incorrect, then the proposed elided “Is it …” translation would be invalid, which would in turn guarantee that the freer “Are there …” translation would also be invalid. I’d opined that the “Are there …” translation is reasonable; although it doesn’t have an identical meaning to the “Is it …” translation, both questions would be used in English to elicit either “eight” or “eighty” as an answer.
mo chara is béarlachas, a literal translation of the English phrase.
In this case the "my" in "my friend" isn't actually a statement of possession, it's a form of address, and the vocative particle a is the appropriate translation. A dhuine might be a more typical usage in Irish, though a chara does work where both people know each other.
The Hiberno-English "your man"/"yer man" (that guy) is mo dhuine:
"Who's yer man?" - Cé hé mo dhuine?