A brief Indian summer is forecast for the next two days with temperatures reaching 27 degrees on Tuesday.
High pressure, which has brought a long spell of dry weather to Ireland since the last week in August, will be followed by a plume of very warm air from Africa, which will settle over the country.
After a misty and foggy start in the northern half of the country, the skies will clear and temperatures on Monday will be between 20 and 25 degrees, warmest in the south midlands.
It will be a muggy night with temperatures between 12 and 16 degrees.
This will be followed by more of the same with the mist and fog clearing to give a day of unbroken sunshine everywhere on Tuesday and temperatures of between 22 and 27 degrees. It will warmest in the midlands.
Temperatures could reach a September record for the century in Ireland, but are unlikely to surpass the 29.1 degrees recorded at Kildare's Clongowes Wood College on September 1st, 1906.
Tuesday, however, will be the last day of the sunshine with rain arriving across the country on Wednesday morning.
Temperatures will remain as high as 24 degrees with the warmth punctuated by heavy showers.
The rest of the week is likely to see a mixture of muggy weather and heavy thunderstorms.