The Irish Blood Transfusion Service is to operate walk-in clinics for regular donors this week, in an effort to avoid an amber alert situation, whereby hospitals would be asked to cancel elective surgeries due to a lack of blood supplies.
The service has issued an urgent appeal, as it needs an additional 5,000 donations - on top of the normal 17,000 - over the next eight weeks in order to meet demand.
It said it has issued more blood than it has collected since the start of June.
The move to boost supplies comes after a pre-amber alert letter was issued to all hospitals last week, restricting issues to emergency orders and patient specific requests.
An escalation to an amber alert would mean hospitals would have to stop elective surgery and use blood for emergencies only.
In a statement, the IBTS said a combination of donors being on holiday, and a rising number of Covid-19 cases, has led to a decrease in supply.
It hopes that the flexibility of a walk-in clinic will enable more donors to attend.
Only regular donors are being asked to attend these walk-in clinics, while new donors should register their interest on giveblood.ie.
Speaking on RTÉ's Morning Ireland programme, IBTS Operations Director Paul McKinney said the service was facing a "really serious situation".
Mr McKinney said that, usually, the service has enough blood on a Monday morning for seven days, but this morning there is only enough for four days.
He said there is a shortfall of 300 units each week over the last two weeks, adding that this looks set to continue and so there is a need to correct it.
Mr McKinney said another reason for the lack of blood was that hospitals were trying to catch up with waiting lists. He also said while there are 65-75% fill rates for clinics, the level of no-shows was worrying.
All donation clinics throughout the country will accept walk-in donors, including the IBTS clinics at Stillorgan in Dublin and St Finbarr's in Cork, which will operate on a dual basis of appointment and walk-in donors.
"We appreciate it may take longer than normal to donate at walk-in clinics during this critical appeal. We sincerely apologise for that and would ask donors to bear with us," Mr McKinney said.
"We feel this is a necessary step to take so we can bolster the supply."