How to go about this would depend, in part, on what specific legal relief the aggrieved party seeks.
Have they sustained actual financial damage or exposure to future potential damage ? If so, the prime culprit is the target for any civil proceedings. However, that could get very messy as if allegations are going to be thrown at the bank they [bank] would need to be co-defendants in the proceedings. This would be based on the established principle that if a plaintiff has proper cause to suspect that the bank has a liability it is not for the plaintiff to elect as between potential defendants as to which is liable and to what degree.
Would rectification of the contractual misdeeds of the miscreant help to resolve this at a practical level ? I suspect not it but it merits consideration as against embarking on legal proceedings with their attendant risks.
On the bare details I have doubts that the FSOP would have jurisdiction over all aspects of this so they might decline to deal with it. Specifically, I doubt that they could be asked to deal with the allegedly fraudulent actions of the miscreant as distinct from the financial institution.
Criminal proceedings are a different matter entirely. That is a matter for Gardaí to investigate. Thereafter, the DPP considers the collected evidence and decides what offences, if any, will be prosecuted. The DPP would prosecute any criminal matter. The aggrieved party would probably be a witness in those proceedings but not a party to them e.g. it would be D.P.P. -v- Freddy Forger .