It is also ending Saturday morning opening at 11 branches.
The bank said it is aiming to make the redundancies "on a strictly voluntary basis".
The banking union, the IBOA, said: "Reducing the number of staff in these crucial roles is not sustainable in the long term."
It added: "It is being driven by a short-term, cost-cutting agenda.
"We are extremely concerned about the impact on customers as well as the increased workload for remaining staff."
Branches ending Saturday morning opening from 14 May are: Armagh, Ballymena, Bangor, Coleraine, Downpatrick, Dungannon, Enniskillen, Magherafelt, Newtownards, Omagh and Portadown.
Seven other Ulster Bank branches will retain Saturday opening hours.
An Ulster Bank spokesman said: "We keep all of our products and services under continual review in response to customer demand.
"This change does not affect branch opening hours during the week.
"Customers will still be able to make use of our online, digital and telephone banking services."
Ulster Bank is shedding around 50 jobs among its branch staff in what it describes as "efforts to build a strong and sustainable" business.