UNITE members at the EBS Building Society have voted overwhelmingly to take strike action during Christmas week.
98% voted in favour of taking action after the company withheld what the union calls a ''13th month payment'' this week.
Last week, EBS offered staff interest-free loans to compensate them for non-payment of the traditional Christmas payment worth a month's salary.
The loans are understood to be subject to benefit-in-kind tax of 12.5%.
Staff were told last week that they would not receive the payment because of rules governing AIB - which merged with EBS in July.
The Department of Finance has said no new bonuses are to be paid at banks.
The union is angry that the payment has been paid only to managerial staff this year.
It says they had the option to change the structure of their pay package some years ago, but the same offer was never made to staff.
EBS had said the payment to lower-paid staff was not pensionable, but managers' payment was part of their salary, which they could choose to have paid over 12 or 13 months, but not a bonus.
The strike action will take place on Tuesday, 20 December.
The union says it is the first phase of what could become more prolonged action.
