The bank said there will be no immediate changes for customers.
Ulster Bank has confirmed a "phased withdrawal" from the Republic of Ireland market over the coming years.
The third biggest lender in the state, Ulster Bank is owned by UK lender NatWest and has 1.1 million customers here along with 2,800 staff.
It's understood its Northern Ireland unit, which also uses the Ulster Bank name, is not included in the decision.
The bank said on Friday there will be no immediate changes for customers, and the changes will come over the next few years.
"Ulster Bank will continue to communicate with customers throughout this process and remains open for business, new and existing through all business channels," it said.
NatWest said it plans to sell a €4 billion portfolio of performing commercial loans to AIB with the staff working on these loans also transferring to AIB.
It also confirmed it is in early discussions with Permanent TSB about their potential interest in buying a number of assets, liabilities and operations.
“Following an extensive review and despite the progress that has been made, it has become clear Ulster Bank will not be able to generate sustainable long terms returns for our shareholders,” said NatWest chief executive Alison Rose in a statement.
“As a result, we are to begin a phased withdrawal from the Republic of Ireland over the coming years which will be undertaken with careful consideration of the impact on customers and our colleagues.”
Ulster Bank CEO Jane Howard said the decision by NatWest to withdraw from this market is hugely disappointing.
"Today will be a difficult and worrying time for our colleagues across the Bank," she said.
"It may also lead to customer questions and concerns as to how this decision may impact them and their day-to-day banking needs.
"I want to be clear that there will be no change for customers today, changes will happen over the coming years. Ulster Bank will continue to offer a full banking service in our branches, online and through normal channels for existing and new customers for the foreseeable future.
"Customers do not need to take any action as a consequence of this announcement. We will communicate with customers in a timely manner over the coming weeks and months."
