What is going on with chàr and càr
Aon (one) causes lenition
The numbers one and two in Gaelic cause lenition on the noun that follows whenever possible:
aon + bàta = aon bhàta (one boat)
aon + piseag = aon phiseag (one kitten)
(Copy of the course tips)
I've seen this on multiple words with the h. I still haven't figured out why.