High levels of tree pollen are forecast in all areas of the country over the next 48 hours, apart from Munster when the level will reduce to moderate tomorrow.
Doctors have warned that people with respiratory problems may have their illness compounded by hay fever over the coming weeks.
Met Éireann says that there is a high pollen risk from birch and ash trees, with a reduced risk on eastern coasts because of onshore breezes.
The increased level of tree pollen at the moment is expected to overlap next month with grass pollen.
The vast majority of people who suffer with hay fever may have mild symptoms such as sneezing, a runny nose, itchy eyes.
Health experts say that, sometimes if there is co-existent asthma, it can set off symptoms of that condition as well with breathlessness, tightness in the chest, a cough, and even wheezing.
There are around 300,000 people in Ireland with both hay fever and asthma.
Hospitals and GPs have recorded a "rebound in all respiratory presentations" post Covid when people generally stopped wearing masks.
