Cancer patients are facing delays in accessing vital treatment because of a national shortage of radiation therapists.
The HSE said the shortage is now so severe that leading cancer doctors are meeting weekly to find solutions, including offering more overtime to staff.
However, trade union Siptu, which represents radiation therapists, said overtime can only be a short-term solution to the shortages.
The union has estimated that 120 fewer appointments are being offered daily.
Radiation therapy offers targeted high-energy X-rays to kill cancer cells and is one of the main treatments for cancer patients.
The union said there is a 30% workforce shortfall compared to demand from sites, including Cork University Hospital.
Siptu sector organiser John McCamley said there are not enough people being trained as radiation therapists.
He claimed that four radiation machines, known as linear accelerators, are not being fully used in Cork, Galway, and Dublin.
“There is a real concern in relation to waiting times,” he said.
"People may work over their hours to deal with cases, and some measures are being put in place, but they are just short-term solutions.”
The Irish Cancer Society has also raised the alarm.
Director of advocacy and policy Rachel Morrogh said people are “psychologically impacted” by delays.
“We know there is a 30% shortage of radiotherapists nationally,” she said.
She said the National Cancer Control Programme (NCCP) aims to have nine in 10 patients meet the targeted timelines for therapy.
“But the most recent data we have shows that around seven in 10 cancer patients are getting treatment on time,” she said.
At Cork University Hospital, oncologist professor Seamus O’Reilly said the crisis reflects a severe shortage of healthcare workers worldwide.
“This is one of those areas where you need a whole-of-Government response to a problem,” he said.
A HSE spokeswoman said: “Time-dependent cancer patients are being treated within acceptable international targets. Patients are reviewed clinically and processed accordingly.
She said targets referred to by Ms Morrogh are “reflective of stresses in the system and not related to outcome of patients”.
There are 205 radiation therapists working across Cork (36), Galway (35), and the St Luke’s Radiation Oncology Network (134).
A CUH spokesman said: “In common with all other healthcare settings, Cork University Hospital is also experiencing a shortage of radiation therapists and is involved in an ongoing recruitment campaign to address the shortage.”