Some 2,000 retained firefighters across the country are returning to the picket lines today.
They are resuming strike action after voting to reject a Labour Court recommendation on pay and working hours, but will cover life-threatening emergencies.
SIPTU has said local authorities have refused to agree an emergency cover plan during the work stoppages, and accused them of pitting the public against the men and women of the retained services.
SIPTU Public Administration and Community Division Organiser, Karan O'Loughlin, told Newstalk Breakfast they will provide the best cover the can.
"The strike started this morning, so effectively every retained station across the country will be closed," she said.
"We are endeavoring to provide as much cover as possible for emergencies, we're not getting a lot cooperation with that from the management side.
"Our people certainly will be available to cover life-threatening incidences".
'A high percentage of a small amount'
Ms O'Loughlin said the offer to increase pay from the Labour Court needs more structure.
"On a percentage basis it looks OK, but it's a high percentage of a small amount," she said.
"Retained firefighters don't actually have wages, they have a small retainer for being available 24/7 - and then they get paid per callout.
"That's quite precarious because they can't rely on that, they don't know how much they're going to earn from one end of the month to the other.
"So we need to get more stability and more structure into that pay, as well as increasing it.
"The Labour Court recommendation just didn't deal with that piece".
'All becoming quite difficult'
Ms O'Loughlin said it is getting harder for retained firefighters to get other jobs.
"It's almost impossible now for retained firefighters to actually get employment where the employer will release them at the drop of a hat, anytime during the day or night, to go and attend an incident," she said.
"If you're out during the night at an incident, you have to get up and be in work on time the following day.
"So it's all becoming quite difficult," she added.
The Retained Fire Service is a 2,000 strong part-time workforce which provides fire and first responder emergency services across the country - apart from county Dublin, Cork city and Galway city, where full-time fire services operate.
