Ambulances from Northern Ireland are "regularly" being used to assist 999 calls in the Republic sparking concerns services are being "stretched to the limit", it has been claimed.
A Sinn Fein councillor has claimed that on one recent occasion ambulances from Northern Ireland were requested to deal with five incidents in Louth in one day.
A cross-border protocol exists between Northern Ireland and the Republic for the two services to support each other during major emergencies and provide "mutual aid".
But concerns have been raised that a shortage of ambulances in the south has led to the Republic "begging" the Northern Ireland Ambulance Service (NIAS) to respond to more calls.
The National Ambulance Service (NAS) in the south has said that "it is not possible to state how many times NI Ambulance Service were requested to assist NAS".
But Louth councillor Tomas Sharkey said he believed there has been a "spike" in the number of calls made in recent weeks for Northern Ireland crews - in particular from the Dundalk area.
"We are begging off the Northern system who are being generous, but it is at the risk of the northern system being overstretched," the Sinn Fein councillor said.
"I am aware of a situation two weeks ago when the northern ambulance service were contacted on five occasions in the one day to come to Co Louth.
"A couple of years ago it was only when an escalated scenario happened - a serious car crash or major incident."
Mr Sharkey said that he did not know whether the calls were Category A - where there is, potentially, an immediate threat to life.
Former Health Minister Michael McGimpsey said he was worried the increasing use of cross-border ambulances could lead to escalating pressures on already stressed crews across Northern Ireland.
"We have a protocol that we support cross-border when there are major emergencies, but it wasn't designed to be a regular service because clearly our service is under extreme pressure. I would be concerned the resources are not going into the service to meet our own obligations in Northern Ireland let alone the Republic of Ireland," he said.
Figures from the NIAS reveal that in the last financial year - until April this year - crews responded to a total of 178,350 emergency calls of which 93 (0.05%) were to the Republic.
A further breakdown revealed 66 were in Donegal, 14 in Louth, six in Monaghan, five in Cavan and two in Leitrim.
It is unclear though how many calls have been made in recent weeks since April 2015.
The NIAS said that although major incidents are an "exception" to the daily running of the service, when they occur they can put "a significant strain on the resources of the service which has the responsibility to manage the incident".
In an NIAS statement, John McPoland explained there are two Memoranda of Understanding (MOU) which outlines the level of "mutual aid" that can be provided between the services.
He said: "As part of contingency planning to ensure that patients, particularly in the border areas, receive the quickest response to provide the highest levels of clinical care at their time of need, the Northern Ireland Ambulance Service and the HSE National Ambulance Service developed a MOU that would enable both services to work within their own procedures and clinical practice guidelines in the other's area of operation."
Mr McPoland said that major incidents are exceptions to the normal day-to-day business of the ambulance service.
"However, when they do occur they can put a significant strain on the resources of the service which has the responsibility to manage the incident," he added.
"In recent years, the major incidents include the fire at a ward block in Altnagelvin and the flooding of the Emergency Department at Letterkenny General Hospital. On these occasions the services were only too glad of the offers of help from their counterparts across the border."