The HSE said three people have died from meningococcal meningitis/septicaemia since late September and have asked people to be alert to symptoms as this usually causes less than 10 deaths annually.
Different regions have reported four cases, and no links have been found between them with three among young adults and one in a child younger than ten.
HSE national immunisation office director Dr Lucy Jessop said early treatment can be effective.
“We have seen pre-Covid five to ten deaths a year but the issue with meningococcal disease it can be very serious and you must recognise it early and get early medical treatment,” she said.
HSE Public Health Mid-West are investigating one case in Limerick which led to the death of a young adult.
Antibiotics are being offered to close or household contacts and vaccines if appropriate.
Meningitis causes inflammation of membranes covering the brain and spinal cord. It can be caused by a variety of different germs, mainly bacterial and viruses.
These cases are bacterial meningitis which is less common but usually more serious and requires urgent treatment with antibiotics.
“Three of them have been confirmed as this serogroup B,” Dr Jessop said.
"We've had much less cases across the year than we had pre-Covid, it is unusual it would happen like this and particularly the seriousness of those cases. We will be keeping a close eye on that.” Last year there were 10 cases compared to 71 during 2019.
Meningitis and septicaemia often happen together. Early symptoms can include headache, neck stiffness, vomiting, discomfort from bright light or fever accompanied by cold hands and feet. A rash which does not fade when pressed with a glass may appear.