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 ?
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?