New figures show that ambulances were held up for over 8,000 hours outside hospital emergency departments in April.
Independent TD Denis Naughten, who sought the data from the Health Service Executive, said the figures are equivalent to having 23 ambulances permanently parked outside hospitals each day.
The HSE national ambulance service has a target turnaround of 20 minutes from arrival time of the ambulance to the clinical handover of the patient.
It said that the crew should be ready to respond to the next incident within 30 minutes.
There were 16,333 ambulance attendances at emergency departments in April.
Of these, 37% were clear within 20 minutes or less, 62% within 30 minutes and 91% within an hour or less.
The turnaround for 8% of ambulances took one to two hours.
Ambulances may be held up for many reasons, including waiting to transfer a patient from the ambulance trolley to the emergency department trolley, cleaning an ambulance after transfer and replenishing supplies.
Stephen McMahon of the Irish Patients' Association said it is vital that people can get to hospital when seriously ill.
"These figures today are very serious. The cornerstone of our healthcare system when emergencies occur is pre-hospital emergency care," Mr McMahon said.
"It's vital that ambulances are available when an emergency happens to someone in the community.
"And if ambulances are tied up in hospitals waiting to transfer patients from the ambulance to the hospital, that means somebody, and patients lives, can be at risk," he added.