The number of confirmed cases of RSV recorded last week is almost twice what health authorities would expect in a normal year.
That's according to the Health Protection Surveillance Centre, which says while the respiratory virus is very common, the illness poses a risk of serious illness to very young children and the elderly.
HPSC Director Greg Martin says the number of recorded cases will be an under-estimation because not all of them will be counted.
"The number of cases that we had notified to us last week was 647. So there were certainly more than that in the community but there were 647 that were confirmed cases that were diagnosed and were notified to the Health Protection Surveillance Centre.
"That's the highest number that we've ever had notified in Ireland, and it's actually almost twice as much as we might have seen in a typical year, so this is this is a substantially substantially higher number than we would have expected."