Around 20 people protested outside employer group IBEC's offices in Dublin at lunchtime in support of workers at Limerick company Curran Aluminium.
They say the company has failed to fully implement a Labour Court recommendation on pay, conditions and union recognition.
SIPTU says 14 of its members have been on strike since October 28 seeking improved terms and condiutions and union recognition.
It wants IBEC, which acted for Curran Aluminium in the Labour Court, to put pressure on the company to comply fully with the recommendation.
It claims that if any companies are permitted to operate on lower terms and conditions for staff, they will undercut compliant firms.
A spokesperson for Curran Aluminium - which does not recognise unions for collective bargaining purposes - said that only five of its current 28 employees are on strike.
It says that an investigation in October by the National Employment Rights Authority found no breaches of relevant employment legislation.
It also criticised SIPTU for its campaign against the company, which it described as 'deeply unreasonable'.
