Dublin, Offaly, Cork and Wexford will see temperatures reach 26C today, according to Met Éireann.
Counties Kildare, Laois and Tipperary will also experience highs of 25C or 26C as the heatwave grips the country.
The forecaster said some areas may hit 28C today, but this heat will be “very localised” and the rest of the country will experience highs of 23C to 24C.
Clare, parts of Donegal, Tyrone and Belfast will see temperatures reach 24C today. Temperatures are currently above average for the month of August.
Meteorologist Paul Downes said the weather advisory, which came into effect today for the entire country, is due to widespread warm temperatures.
Mr Downes said there is quite a shift between daytime and nighttime temperatures, but overnight highs will increase over the coming days to make for uncomfortable sleeping conditions.
The forecaster said a heatwave consists of temperatures above 25C in the same location for five consecutive days.
It added that the impacts this hot weather will have is heat stress, especially for the more vulnerable of the population, a high Solar UV index and a risk of water related incidents.
Temperatures will widely reach 25C across the country tomorrow with highs of 28C possible.
Mr Downes said the Status Yellow hot weather warning, which comes into force at 12pm tomorrow, will see nighttime temperatures rise in Leinster and Munster and beyond.
The UV index will also be quite high this week as there will be less cloud cover and more sunshine.
Mr Downes said there is still some uncertainty in the forecast from Sunday onwards- with a risk of thundery showers but that warm conditions may continue into next week.
“But the latest forecast is actually keeping quite warm temperatures through the early days of next week as well, possibly 26C or 27C in parts of the east on Monday,” he said.
“More low to mid-twenties on Tuesday and Wednesday, there is potential there for quite warm temperatures to continue.”