I didn't know the distinction either. Thanks for that.

In fact, I highly doubt there are any Hiberno-English speakers left at that rate. Certainly there are natively bilingual Irish and English speakers, but I don't think there are any native Irish speakers who learned English as they got older. The days of Micky McGowan and Rothaí Mór an tSaoil (translated as "The Hard Road to Klondyke" excellent book for those interested in Ireland at the turn of the last century) are gone.

Irish English is old-fashioned. I would rarely use the grammatical constructions of it (with a week, he does be), but there are a few I do use. Many members of my parents' generation use the vocabulary, and even I would use some of it without noticing. I also noticed that we do tend to reply to questions with the verb, (Did you, I did etc.) which I presume is an influence from Irish.

It certainly is in my case. I noticed myself doing this after my last bout of intensive Irish study. I've also recently become aware of a shift in my emphatic constructions from "He's the one who..." to "It's him who...", presumably under influence of "Is eisean a...".

Do you ever use "I'm after Xing"? I have various reports that that construction is still current, at least among speakers of roughly my generation or older.

I don't think I use it a lot personally, but certainly phrases like "I'm after doing/saying it" doesn't sound strange to me at all, indication that I hear the phrase pretty regularly. Phrases that spring to mind are, "I'd know" presumably from N'fheadar, and people go in funerals and weddings. Looking at the Wikipedia article, I often end sentences in "isn't it" or "doesn't it" or yeah or no depending on the answer I expect. Or, "I'm doing the right thing, amn't I?" Shur, or ahshur and those sort of interjections are extremely common, and I never realised that "this one here" or "that one there" was an Irish phenomenon. Also, bring and take seem to correspond to what wiki says is the Irish way. Ye is what I would use in speech for the second person plural pronoun, and yere. "I've my homework done" sounds perfectly normal to me too, is that not so for other dialects? And the myriad of insults and terms of endearment, I'd certainly use some of them, but the others are probably limited to older generations. Messer, Plaidhce, Rogue, Amadán, Oinseach, lúdramán, craytur, mo ghile, mo ghrá. Then plamás for nonsense or enticing hollow words.

I can't speak for the Commonwealth, but that sounds completely out of character for American English. "I've got my homework done" barely passes muster, but lacks the familiar ring of "I got my homework done".

Where I'm from, "I've my homework done" does not sound natural at all. If someone said that, they'd probably turn a confused head or two. "I've got my homework done" would be acceptable, however it would probably come out more like "I got my homework done," and would imply that the speaker has completed their homework at a previous time. Example: "Hey, you wanna' go see a movie?" "Yeah. I got all my homework done so I'm free. Let's go." If one wanted to express that they had just freshly finished their homework, then one might say "I finished my homework," but would probably be more likely to say something like "Homework's done," or simply "It's finished," or even just "Done." Again, I speak for the people where I live, and it's worth noting that my region does have a rather heavy AAVE influence. Although I'm not sure if that has any effect on the phrases I've listed here.

I'd just say "my homework's (all) done".
Once in a while you might catch me saying "I'm after Xing", but that's not because it's in my dialect of English, it's because I think it's funny to speak in a word-for-word translation of Welsh. (I sometimes say "don't with X" for the same reason.) I didn't have any idea that that use of "after" was actually present in any dialect of English.

Actually, now it's just becoming a sort of habit for me. Ever since I started learning Welsh sort of seriously, I've fallen into some weird (for my native dialect) habits of speech, and "after Xing" is one of them. There's also "this one here/that one there", "there is X with me" and sticking predicates at the beginning of the sentence. The last one is somewhat related to the constructions exemplified in "the cat's in the house she is" and "that's not true it isn't", which as far as I know are pure Tikolm-isms. (If I find myself saying "wasn't she cold this morning here!" [for "was it ever cold this morning!"], then I'll know I'm really in trouble. )

Anyway, back on topic, if I can find the topic anywhere. "It'sn't" doesn't work because "it" here isn't a stressed pronoun and it would have to be stressed in "it'sn't".

I would understand the constructions sans do-support, but then again, I'm not a regular Anglophone.

What play I? I the French Horn play.

I've just gotta point out, that the reason I think you folks are having trouble with it'sn't is because the contraction is probably 'tisn't, from the archaic 'tis (it is). I've heard people use it, but usually only when they're trying to imitate Victorian English (and poorly at that), and not in everyday usage. 'Tisn't unpronounceable, however!

Should be /ˈtɪzənt/ for most people.

That said, I don't know of any case in English where you'd intentionally contract three words into one.

As for it's not vs. it isn't, like someone said before, it's simply a matter of emphasis; a non-grammatical way of denoting case. At least, that's how I'd use it.

Doncha?