The Health Service Executive has reported an increase in meningococcal disease.
It said 11 cases were notified to the Health Protection Surveillance Centre (HPSC) since the last week in December.
Three patients diagnosed with meningococcal disease have since died.
All three deaths were directly due to the infection.
The HSE said meningitis and septicaemia can often happen together and symptoms can appear in any order. Some may not appear at all.
Early symptoms can include; fever, headache, vomiting, diarrhoea, muscle pain, stomach cramps, fever with cold hands and feet and a rash.
The HSE said all age groups have been affected, ranging from infants to the elderly.
Provisional data on the strain types identified since the last week in December indicates that different strains of the organism are in circulation.
Of the three patients who died, two different strain types were identified.
The 11 cases notified to the HPSC between the last week in December and the first week in January compares to five cases over the same time period last year.
In 2018, a total of 89 meningococcal cases were notified compared to 76 in 2017.
The HSE has advised the public to be vigilant and said parents of children should ensure that they have received all of their vaccines on time.
Dr Suzanne Cotter, Specialist in Public Health Medicine, HSE Health Protection Surveillance Centre said: "Although meningococcal disease incidence generally increases in the winter months, the recent increase is cause for concern and the HSE wishes to alert the public to the signs and symptoms of this disease so that immediate medical attention can be sought if someone has symptoms that could be caused by this bug."
Dr Cotter said anyone with concerns should contact their GP in the first instance.