Stricter EU limits means hospitals may have a shortage of blood supplies
Irish blood supplies may not be maintained over St Patrick's Day and Easter, the transfusion service has warned.
Hospitals have been asked to conserve the use of some blood groups and reserves are down to four or five days.
The problem is due to an increase in the number of donors turned away, the provider said.
New EU donation limits on haemoglobin levels mean the service is rejecting a quarter of potential donors.
Irish Blood Transfusion Service (IBTS) director Paddy Bowler said: "This morning, blood supply levels of the main blood groups stand between four and five days.
"While the knock-on effect of low haemoglobin levels is expected to last for some time, it is imperative that donors come forward over the next few weeks to safeguard blood supplies during the extended holiday period of St Patrick's Day and Easter."
While donors have been attending in their usual numbers over the last six weeks, the amount donated remains low due to many having a low haemoglobin measurement.
Mr Bowler said up to October one in seven donors attending clinics were unable to donate for various reasons but since then this has gone down to one in four.
"The increase is caused entirely by donors presenting with haemoglobin levels below permitted EU donation limits."
In recent weeks the IBTS has requested hospitals to conserve the use of blood groups O negative and B negative.
The transfusion service has scheduled six additional mobile clinics for March 20.