Siptu has served strike notice on Shell Ireland on behalf of 65 staff in Dublin, Galway, Limerick and Cork.
The workers claim their conditions of employment will be eroded in the takeover of Shell's Irish business by Topaz Distribution and Logistics.
In particular, they say the new pension scheme offered by Topaz is less favourable than the existing one and that workers will no longer be offered share option schemes because Topaz is not listed.
Most of the 65 workers are employed at Shell Ireland's head office in Clonskeagh in Dublin.
Siptu's Ann Ryan says Shell Ireland is attempting to impose change without agreement that will result in a worsening of employment conditions for its members.
"We have a long-standing agreement, dating back to 1985 which requires both parties to maintain the status quo until the Labour Court has had an opportunity to examine the issues in dispute and to issue a recommendation," she said.
"Despite our procedural agreement, Irish Shell - which is a hugely profitable company - has flatly refused to await the outcome of the Court's deliberations before attempting to dismantle terms and conditions which will see our members lose out on a five-year pension credit; an early retirement option at age fifty and a defined benefit pension scheme."
The company is due to release a statement later this afternoon.
In July Topaz Distribution and Logistics - a consortium fronted by former Emo Oil managing director Danny Murphy - won the takeover battle for Shell Oil's Irish retail and commercial business.
The value of the deal agreed with Royal Dutch/Shell was not disclosed but the deal was thought to have been worth at least €200 million.
