Very strange indeed. Let us know the old and the new rate; the amount outstanding; and the old and new payments.
Could it have coincided with the expiry of Tax Relief at Source on the mortgage interest?
Are you in arrears by any chance? While this should not matter, Irish Nationwide made a systematic error on their calculations which resulted in exactly what you are describing. "Dear John , we have reduced your interest rate by 0.5% . Your new repayment will be €x ." which was higher than the old payment.