The strike by 200 of Bank of Ireland's IT staff in protest at the bank's outsourcing plans is under way.
The bank and the Irish Bank Officials Association have both pledged that the dispute will cause minimal disruption for customers, although there are signs the one-day strike could escalate.
The staff involved work at ITSIS, the bank's information technology support division, at Cabinteely, Co Dublin, provide support for the bank's key services such as ATMs and Internet banking.
Some 200 of the 300 staff at ITSIS are picketing their headquarters at Cabinteely, Co Dublin, in protest at the terms under which Bank of Ireland wants to outsource work to Hewlett-Packard.
On the eve of the dispute, IBOA general secretary Mr Larry Broderick said its members were very responsible people who would show consideration for Bank of Ireland customers and their colleagues.
But he warned the trade union could seek to escalate industrial action across the wider Bank of Ireland group unless the issues are resolved.
A Bank of Ireland spokesman said the bank had contingency plans to limit the impact on customers. "It will not be evident to the general public, but there will be disruption internally," he said.