Siptu is describing as unacceptable that ambulances are being delayed for up to seven hours outside emergency departments.
It says the delays are happening because patients are not being discharged from ambulances into the care of nursing staff.
It is demanding that the Health Minister, HSE and Department of Health take immediate steps to relieve pressure on its members.
Siptu Divisional Organiser Paul Bell says ambulance workers are being held up at EDs instead of being able to serve communities.
"At the end of November, Siptu representatives requested that the HSE and Department of Health agree a protocol for the handover of patients at emergency departments. Unfortunately, our calls were ignored and now we have an unacceptable situation where our members are reporting delays in some cases of between three and a half and seven hours outside emergency departments as our now annual winter overcrowding crisis bites," Mr Bell said.
"While the HSE and Department of Health are responding to some areas of the overcrowding crisis, primarily by attempting to boost the number of beds available in hospitals there seems to be little consideration or emergency planning to make sure ambulances are kept on the road and readily available for communities."
Mr Bell highlighted "absurd" incidents over the weekend, including crew members undertaking a 901km roundtrip during a 12-hour shift and ambulances from Kilkenny being dispatched to Cork.
"Over the weekend, we had the absurd situation where Siptu members working a 12-hour shift in an ambulance base in County Clare were dispatched on a 901km roundtrip to Clonmel and back to Youghal due to local resources in being held up in Tipperary while ambulances from Kilkenny bases were dispatched to emergencies in Cork.
"This chaotic system is not only bad for patients and driving up ambulance waiting times it is also having detrimental effect on the health and wellbeing of our members with many ambulance professionals continuously exposed to long shift over-runs and unsatisfactory rest and break times."