Ulster Bank has confirmed its intention to close five branches and six sub-offices in the Republic of Ireland by May.
The bank will close seven branches and four sub-offices in Northern Ireland by June.
The branches set to close are Belturbet, Co Cavan; Castlepollard, Co Westmeath; Glenamaddy, Co Galway; Killeshandra and Kilnaleck in Co Cavan.
Sub-offices at Carrigallen, Co Leitrim; Delvin, Co Westmeath; Kilcormac, Co Offaly; Kilkelly, Co Mayo; Rathangan, Co Kildare; and Swanlinbar, Co Cavan, will also close.
In a statement, Ulster Bank said that customers' accounts in the Castlepollard branch will transfer to Mullingar, Killeshandra accounts will transfer to Arva, Kilnaleck will transfer to Ballyjamesduff, Belturbet will transfer to Ballyconnell and Glenamaddy will transfer to Tuam.
The bank said that it will be contacting customers of these branches to inform them of alternative branch locations in their area.
The Northern Ireland branches set to close are Carryduff near Belfast; Dromore, Co Tyrone; Harryville near Ballymena; Jordanstown near Belfast; Knock near Belfast, Longstone Street in Lisburn; and Shaftesbury Square in Belfast City.
Sub-offices at Ardglass, Co Down; Moy, Co Armagh; Rosslea, Co Fermanagh; and Saintfield, Co Down will also close.
The Irish Bank Officials Association has expressed disappointment at the move.
IBOA General Secretary Larry Broderick said that even though speculation had been mounting in recent weeks, today's announcement by Ulster Bank management specifying the number and locations of the branches earmarked for closure will raise a number of concerns for both customers and staff in these areas.
"Staff throughout Ulster Bank have worked strenuously to restore the bank's reputation, especially in the wake of the catastrophic IT malfunction last year, which created major difficulties for hundreds of thousands of customers. They are naturally concerned about any developments that might undermine customer confidence," Mr Broderick said.
However, the IBOA welcomed Ulster Bank's commitment that no further branch closures are planned for this year, as well as confirmation that no additional job losses will be necessary beyond the figure of 950, which was announced last year.