You made a complaint to the bank.
Some appropriate person rejected it.
You asked for a final complaint letter.
Before it was sent a senior person reviewed it and didn't agree with your complaint.
You went to the FSO.
Correspondence ensued so it's likely that someone else senior in the bank looked at it.
The FSO rejected your complaint.
It's extremely unlikely that the High Court would overturn the complaint.
You would need a top quality solicitor and barrister who is interested and who understands your complaint. They are not easy to find.
The FSO uses a very good barrister who is experienced in handling all of their appeals.
I am aware of one case where the FSO found in favour of the customer. The Bank appealed to the High Court. When the FSO's barrister looked at the case, he said that the FSO's decision was indefensible. So they did not contest the appeal. The FSO heard the complaint again from fresh and found against the customer. I suppose that could happen, but you are risking €300k on a 50/1 shot.