It's reported flu cases are likely to peak during Christmas week.
DCU Professor of Health Systems Anthony Staines says the flu surge is "potentially quite serious."
He's advising people to get vaccinated as soon as possible to prevent the spread of the virus.
Professor Anthony Staines says it's hard to know why it's so bad this year:
He said: "It is a different strain to the strain that we had last year.
"It is even a different strain to the strain they had in Australia during the summer.
"We don't know much about it. People don't have much immunity to it, necessarily," Prof. Staines added.