The Ulster Bank 2.3% is for 2 years. So a comparison over 2 years would be (seems to me):
UB €250,000 * 0.023 * 2yrs = €11,500 interest and then minus cashback of €1,500 is €10,000. So 'costs' that much.
BOI €250,000 * 0.03 * 2yrs = €15,000 interest and then minus cashback of €5,000 (2% of...