SIPTU has said it will escalate a dispute over pay at GMC Civil and Mechanical Engineering following the failure of talks aimed at resolving the matter.
The union said pickets will be placed at GMC depots across the country and that representatives will meet with shop stewards in Gas Networks Ireland on Friday morning to agree supportive action.
GMC is contracted by Gas Networks Ireland for the installation, maintenance and provision of emergency response on the gas network.
Last week, SIPTU members at GMC took strike action accusing management of failing to implement agreed recommendations on pay and conditions.
Pickets were suspended on Friday after both sides agreed to attend the Workplace Relations Commission (WRC) but those talks ended last night without agreement.
"The company is not serious in its intent to resolve this dispute," said SIPTU Divisional Organiser Adrian Kane.
"SIPTU will also be seeking an urgent meeting with Gas Networks Ireland. Ultimately, GNI must take responsibility for the conduct of its principal contractor," Mr Kane said.
SIPTU said it will be insisting on the insourcing of work in its meeting with GNI.
GMC has previously said it has engaged constructively with employees and their representatives and has not frustrated any resolution of the pay dispute.
The company said it is a member of the Construction Industry Federation and that all employees were paid in accordance with the Construction Sectoral Employment Order.
Gas Networks Ireland said is working with GMC to ensure that contingency arrangements are in place to manage any emerging issues on the gas network.
"Safety is our number one priority and because of these measures being put in place, we expect no disruption to the continuity of gas supplies on the network," a spokesperson said.