Why does amazon.ie redirect to amazon.co.uk, as import taxes now apply

If they organise their logistics so that orders to Ireland are shipped from warehouses in Ireland (either the Republic or Northern Ireland) then there is no import duty or any additional formality.

They can't do anything about sellers located in Great Britain and handling shipping themselves, and I suspect that there is little to be done in any case because Ireland is quite small and right next the the UK... Maybe just filtering listings so that only Ireland-based ones are shown if the visitor so decides.