Organisations with defibrillators are being urged to ensure the devices are checked and are working correctly.
The Health Products Regulatory Authority says it is vital that the recommended safety and maintenance updates on the devices are carried out.
The HPRA has identified around 940 defibrillators in Ireland where a "corrective action" remains outstanding.
It says updates are needed immediately on these devices to ensure that they will work in a life-saving situation.
It warns that weather temperatures will affect a defibrillator's performance and it says all devices should be regularly checked during the winter months.
There are an estimated 5,000 sudden cardiac deaths in Ireland each year with over 70% happening outside a hospital.
Spokesperson for the HPRA Anne Tobin says: "It is estimated that there are some 10,000 AEDs in Ireland.
“We know that almost 950 of these have the potential to not work effectively in an emergency because a corrective action as deemed necessary by the manufacturer has not been completed.
“We know that the manufacturers concerned have attempted to contact the owners directly with some also using national advertising to highlight the importance of carrying out the required upgrade or battery check."
The 940 AEDs where corrective actions are needed to ensure they work effectively are Lifepak CR Plus and Lifepak 1000, both made by made by Physio Control Inc, AED Plus manufactured by Zoll, Samaritan PAD 300, 300P and 500P, which are made by HeartSine.
