Fórsa says local authorities will experience a "summer of strife" unless they negotiate with them.
Members of the union, which represents over ten thousand local authority staff across the country, have voted overwhelmingly in favour of industrial action in a row over job evaluations.
Fórsa says the disagreement began during the financial crisis when thousands of jobs were lost and staff were given extra duties above their pay grade.
The union claims the Local Government Management Agency, which represents the country's 31 local authorities, has refused to negotiate with them on the matter.
