Almost 40 security staff at the Corrib Gas Terminal in north Mayo have begun industrial action in a dispute over payments and terms of employment.
The personnel involved in the one-day stoppage at the Bellanaboy facility are all members of SIPTU.
Those taking part in the dispute are employed by the Senaca Group, a security services firm, which is contracted by Shell E&P Ireland to safeguard its operations in Co Mayo.
SIPTU has said the workers are taking the action after a number of contracts were terminated and working hours were unilaterally reduced in recent weeks.
It said the company has refused to attend the Labour Court for arbitration on other issues, including local operational practices, functions around first-aid provision and mustering - a safety practice in place on the site.
Union members will start a work-to-rule after today's work stoppage.
The Senaca group has said the completion of the construction phase of the Corrib gas project always meant it would have to scale back its operations in Bellanaboy.
The company says it has been trying to manage this on a phased basis since 2013.
In a statement this evening, Senaca said that in the absence of an agreement with SIPTU, it had adopted a "last in, first out" approach to compulsory redundancies.
The company says the union has rejected two LRC proposals since last July and that SIPTU did not refer redundancy proposals to members so they could ballot on them.
It says it regrets the industrial action and is still hopeful that agreement can be reached with staff and union representatives.