SIPTU members working in water services in local authorities across the country will take strike action on 7 and 8 June.
The move could lead to significant disruptions to water supplies.
The dispute relates to the transfer of water services from local authorities to Uisce Éireann.
SIPTU said that the objective of the action is to ensure that workers not transferring to Uisce Éireann but remaining in local authorities will not suffer any loss of earnings.
The union claims that significant amounts of money will be lost by water workers on a weekly basis and that there will be a major impact on their pensions if the matters are not addressed.
"There will be a two-day strike on 7 and 8 June followed immediately by a series of industrial actions," said Stephen Kelly, water worker and chair of the SIPTU national negotiating committee.
"It is unfortunate that it has come to this, but water workers have to ensure that they will not be at a financial loss should they choose to remain with their local authorities."
"While we have been given commitments in relation to regular/rostered overtime, there is no such confirmation on allowances," Mr Kelly said.
SIPTU said that once the dispute commences, it is likely that there will be water shortages and boil water notices as the ongoing quality of water cannot not be monitored in the normal way.
The union said that the action will start with a two-day stoppage, followed by rolling industrial actions and more strike days cannot be ruled out.
SIPTU has accused local authorities of taking the narrowest possible view of what the retention of terms and conditions means, thus creating a significant financial loss for water workers.
The Department of Housing, Local Government and Heritage said that water services staff who choose not to voluntarily transfer to Uisce Éireann will remain in the direct employment of their current local authority maintaining their current terms and conditions of employment.
"They may continue to work in water services, under the direction of Uisce Éireann, until 31 December 2026," a spokesperson said.
"Local authorities will no longer have any involvement in the direct provision of water services to the public beyond that date.
"Further opportunities for staff who do not seek to transfer to Uisce Éireann will be identified during the transition period to 2026, including reassignment, redeployment, upskilling and training programmes.
"Under the framework, these workers will suffer no loss of earnings relative to their current water services role after they are reassigned within their local authority."
The Department said that local authorities will work with staff who are to be reassigned to local authority roles outside of water services, adding that this is expected to be a relatively low number of the overall water services workforce.
A voluntary redundancy scheme will also be made available to eligible local authority water services staff members.
The department said that a framework document for workers sets out the options that will become available generally and empowers them to decide voluntarily on the option which is best suited to their own individual needs.
