It was caused by a computer failure in the overnight processing of account information, and also led to online banking and payment problems for millions of RBS customers in Britain when a software upgrade went wrong.
Ulster Bank said it accepted the findings. It has paid £46m (59m euros) in compensation to affected customers.
It is likely to face a further substantial fine from the UK's Financial Conduct Authority in relation to the Northern Ireland part of the bank .
The Central Bank's investigation found a major failing in the outsourcing of Ulster Bank's IT systems, claiming that Ulster did not have robust governance arrangements in place.
The central bank said its findings had been accepted by the bank as part of the settlement agreed between the two. It said the fine was the highest imposed to date under its Administrative Sanctions Procedure.
Directer of enforcement, Derville Rowland, said:
"The summer of 2012 saw an unprecedented disruption to banking services. The major breakdown as a result of IT failings is completely unacceptable."
The central bank's head of credit institutions supervision warned on Tuesday that banks whose IT systems are already under strain will need to make significant investment if they want to do more of their business electronically.
