I think that the hatred of ginger people is most likely a result of not as prevalent but still existent irish racism
It's historically older than a lot of anti-irish sentiment (Shakespeare for example wrote gingers as evil).
But there's probably some level of anti-celtic views in the historical abuse towards gingers, as the largest proportion of ginger hair in the UK is found in Scotland, followed by Ireland.
But most abuse in the UK towards redheads today is probably not based on the Irish connection.