That would work, too. Remember that this is still beta and not all correct answers are accepted, yet.
gluaisteán is the older word for car but carr is used more these days. Both are correct
Carr is in Dinneen’s 1904 dictionary; gluaisteán isn’t. Both are indeed correct.
Carr is actually béarlachas whereas gluaisteán is the original Irish word for car. By the 19th century English was the dominant language in many areas, perhaps this explains the absence of gluaisteán in Dinneen's 1904 dictionary.
Presuming that carr comes from English “car”, the etymology of English “car” goes back to Proto-Celtic. (It was only later in the 20th century that both words acquired the primary meaning of “motorcar”.) I’ve looked on eDIL, but couldn’t find a reference to gluaisteán there. What is the earliest reference that you’ve found to gluaisteán ?
My apologies, in school I was always told not to use carr as it is béarlachas and a few websites have also mentioned this. You are right, carr comes from chariot or cart while gluaisteán appears to have been introduced for automoblie.
Ó Dónaill translates 'carr' as car and 'gluaisteán' as motor-car. Does that mean that 'gluaisteán' is archaic, since 'motor-car' surely is nowadays?
No — it means that carr can also be used for other meanings, e.g. a rail car, while gluaisteán can’t.
I could be wrong but I don't think gluaisteán is archaic. In school our teachers preferred us to use 'gluaisteán' instead of 'carr' and 'guthán' (telephone) instead of 'fón' as they claimed those words were "more authentic and less English sounding/influenced."
I heard someone say gluaisteán on the radio, so I'm going to stick with that, although I don't know if duolingo excepts it
When I look up the Wikipedia page for the IPA representation of Irish phonology/orthography, I can see that there is/used to be a distinction between double consonants and single ones, namely that "nn" and "ll" were more dental than their single counterparts. Did this apply to "rr" too, or was "rr" pronounced more like a trill than a flap?