A Health Information and Quality Authority review of the ambulance service has found that there are significant shortcomings in Dublin, putting patients at a high level of risk.
HIQA also found that the national service still lacks necessary capacity and despite increased recruitment rates, remains reliant on overtime to maintain services.
The report says that more has to be done to ensure a modern, effective emergency ambulance service is provided by Ireland's two publicly funded services - the National Ambulance Service and Dublin Fire Brigade.
It says that, currently, if a patient with a potentially life-threatening condition in Dublin calls 112/999 for an ambulance, arrangements for call handling and dispatch can result in a delay in response.
This is due to the process for transferring calls from Dublin Fire Brigade to the National Ambulance Service, where the DFB has reached operational capacity.
Also, a Dublin Fire Brigade resource may continue to be dispatched to such a call in a situation where a nearer National Ambulance Service resource may have been available and better placed to respond.
In its review, HIQA said that if a patient or member of the public rings 112/999 for an emergency ambulance in Dublin, there is no means of ensuring that the nearest available appropriate resource from either service is always immediately sent to answer the call, regardless of whichever agency is providing the resource.
Neither Dublin Fire Brigade nor the National Ambulance Service has sight of the location of each other's resources, as they operate different and separate computerised dispatch systems which are not linked together.
The health watchdog says that while lines of communication and formal governance arrangements at senior management level within the HSE and Dublin City Council have much improved, there is still no detailed plan for the delivery of emergency services in the greater Dublin area.
The review calls for a significant increase in staff and a long-term commitment to finance this increase in order to meet response time performance levels.
Today's 124-page report is a review of progress in implementing the recommendations of HIQA's 2014 review of pre-hospital emergency care services.
It says that key improvements have occurred, in particular the National Ambulance Service move to a single control centre over two sites.
But a key finding relates to the service in Dublin where there is a "high level of risk" associated with a lack of collective ambulance capacity and arrangements for call handling and dispatch.
Up to 2005, services were provided by nine providers.
The National Ambulance Service covers most of Ireland and the Dublin Fire Brigade covers most parts of Dublin.
The report says that National Ambulance Service funding has been increased by 9% since 2014 but funding provided by the National Ambulance Service to Dublin Fire Brigade to part-fund ambulance services has remained unchanged since then.
Both services have experienced a year-on-year increase in the demand on services.
Dublin Fire Brigade receives an average of around 320 emergency ambulance calls a day.
In a statement, the Health Service Executive welcomed the review saying it appreciates HIQA's acknowledgement that significant changes have occurred to address the recommendations outlined in the 2014 report.
It said these changes have come about as a result of the continued commitment and dedicated work of staff within the National Ambulance Service.
SIPTU Divisional Organiser Paul Bell said that the service needed extra investment to make it effective in city and rural areas.
He said the HIQA report showed there had been progress and that SIPTU members in the National Ambulance Service and in Dublin Fire Brigade had contributed to ensuring improvements since the 2014 review.
Meanwhile, the National Ambulance Service Representative Association has said the service needs more investment and staff to meet growing demand on services.
Its chairman, Michael Dixon, said it was extremely difficult to attract recruits when pay and conditions have been progressively eroded.
HIQA said the National Ambulance Service now has a clear understanding of what it needs to do to progress services, "aided through a series of reviews examining capacity, workforce development and fleet management."
The HSE also said that the National Ambulance Service is in the second year of a five-year strategic plan.
Damien McCallion, the National Director for the service, said the HIQA report found that lines of communication, formal governance arrangements and working relationships at senior management in the HSE and Dublin City Council were much improved.
He welcomed the report and said more needs to be done in some areas of work.
Mr McCallion said that significant changes have occurred since the previous HIQA review in 2014.