I've just been re-reading earlier contributions to this discussion. I wonder just how much evidence there really is, archeological or literary, for the use of chariots in Ireland (a) for travelling, and (b) in burial rites with horses. It seems that Barry Raftery once said that remains of roads designed for chariot traffic have been found in Ireland, but I wonder how he knew that they were designed for chariots rather than any other kind of vehicle. It is also said that Raftery said that "Irish chariots were made of wicker, which quickly dissolved in soil, so none have been found": but if none have been found, how did Raftery know that they were made of wicker, or even that they existed at all? Did Old Irish have a special word for "chariot" as distinct from "cart" or "wagon"?
Even if chariots existed in Ireland, the existence of the burial rite is a different question altogether. After all, the Romans had chariots, but they didn't bury emperors in them. I find it hard to believe that the Welsh chariot burial had any kind of cultural connection with Ireland.