I have a BOI lodgement slip in front of me (I assume that what I want to do, ie a mortgage redemption is processed as a normal "lodgement") there is no space on the slip for a narrative.
That said if it was possible to include a narrative it would be my 8 digit reference number (what a silly thing for them to do to use 8 digits, makes it look like an account number) I wonder how many people mistakenly pay money into this reference number thinking it is their account. Could be a nice earner for the bank, as the daily interest is acrued daily, and any problem might take a week to redirect.