Just to make something clear, although there is little archaeological evidence of chariots in Ireland there is to much to ignore in the early christian/medieval laws and myths to say that they were never used. I believe (I don't have my books in front of me) that one of the names for an Irish caste was 'chariot rider', for example. Its just my personal opinion that the use of chariots has been overstated in the tales, and that calvary was much more common.

As to the climate of Iron Age Ireland, it was a bit wetter and colder than modern day Ireland. The biggest difference though was that it was covered in forests.