There are a number of possible explanations, but the most likely is along the following lines:
After you moved in, your solicitor sent transfer deed to Revenue for stamping. Then your solicitor sent transfer deed to Land Registry. If it was a new housing development, it would not be unusual for it to take two years for registration to have been completed. If there were queries raised, it might have taken longer. Eventually, when all registrations were completed, your solicitor put together all the title documents and sent them to your lender. At this point, your solicitor was ready to close the file and send it out to archive\storage. At this point, your solicitor noticed that he\she had some money in client account belonging to you. Perhaps your original account included a provision of €500 for land registry fees but they instead came in at €85 ( an error which can arise where the unit consists of a brand new unit held on a new 999 year lease; I mention this specific error because I have done it myself on a bill; heck, maybe it was me who sent you the €400!).
It could also be that the file was in fact finished and closed a couple of years ago and that the credit balance on your ledger ( in the solicitor's client account) was later spotted and queried on the annual audit.