No idea how old this thread is, but there is, actually. Southern American English dialects are heavily influenced by Irish and Scottish constructions and accents and are actually closer to some traditional speech patterns than standard American English. It's one of the fun things for Southerners to learn when studying how we got from Old English to American English because it means there is an actual reason for the way many people speak rather than just laziness. What Americans refer to as the Southern drawl and "hick" usage is actually what the Irish, Scottish, and northern English accents and speech patterns evolved into when mixed with the others around them, and it just became the sound of the area as time went on. We spent quite some time in my History & Structure class just talking about the overlaps. My 7-yr-old English cousin thought my friend was "speaking Irish" when talking, but she just has a strong Southern accent at times and uses a lot of nonstandard dialectic constructions.
For idiomatic speech, the Gaelic languages (Irish, Scottish Gaelic and Manx) absolutely love negative constructions. It's just part of this branch of languages. My favorite is how some Scottish Gaelic speakers say ceud ach aon (100 but 1) for 99. It's just more fun than naochad 's a naoi (the decimal form) and easier than naoi deug air ceithir fichead (the vigesimal form-- lit. 19 on four 20).
The Connacht pronunciations of deartháir and deirfiúr are confusingly similar - easy enough to tell them apart when you hear them together, but potentially confusing when encountered on their own.
You can hear more examples on Track 25 of CD1 from the Routledge Colloquial Irish, which asks questions about brothers and sisters:
I can't for the life of me make out how deirfiúr is pronounced based on this audio and the one I just heard before (the plural in lesson Family 1), my ears cannot split this word apart (I hear "tsi-rawn"??). What is going on :( teanglann offers something completely different as well. Should I trust duolingo's audio bit?
edit: this one sounds way more clear https://www.duolingo.com/comment/13520185
I still would like to know what happened in the above sentence in terms of pronunciation.
Huh. She clearly is pronouncing this one word is two different ways and for no reason I can see. The pronunciation is in the second example makes sense to me, but not this one.
I have suspected the person who recorded the new audio is Brid Eillis, also known as "Brid Mhór" on the Irish Learner's Forum. I have listened to a number of her recordings on both Forvo and IRF, and subjectively speaking (and no offense to her), she is not someone I want teaching me Irish pronunciation. Her pronunciation is often unclear and difficult to understand, and even for a Connacht speaker, her idiolectial variations seem pretty inconsistent and much more outside of the kind of Irish I want to learn. I want to learn a standard, more neutral Irish I can be comfortable speaking, not a sad mimicry of one speaker's curious idiolect. But that's just me, and some folks may think I'm being harsh.
Anyhow, she has a recording of her saying deifiúr alone on Forvo: http://forvo.com/word/deirfi%C3%BAr/#ga
Compared to the other speakers, you notice she pronounces the initial slender d much harder and more t-like than other speakers and she does not stress the first syllable. However, this is unlike the either example in these two Duolingo sentences, so I don't know what to tell you regarding why this speaker does this. From my experience with Gaelic and Irish pronunciation, I would be inclined to pronounce this word like the Munster speaker on Teanglann or the other Forvo speakers (the Ulster speaker on Teanglann adds an "intrusive vowel" between the r and f which is something that sometimes happens with consonants following a r, so that is not really that unusual).
She's not saying "deartháir", she's using a Connacht pronunciation for deirfiúr, which is similar to, but not quite the same as, the Connacht pronunciation for deartháir (too similar for me to tell them apart unless I can hear them side by side, but quite distinct when heard together).
I've read that you have to use a different counting system when talking about people. Not sure if, or in which cases, there may be any exceptions. I think you would also have to change the sentence to a "positive" statement, I think. My guess would be, "tá duine deirfiúr agam," but don't take my word for it until one of the more qualified members confirms it.