Twenty-one counties have been hit with a Status Yellow thunderstorm warning.
Met Eireann has issued the alert as scattered thundery showers are set to lash most of the country throughout Monday afternoon.
The thunderstorm caution will be valid between noon and 9pm on Monday.
It will be in place for Cavan, Monaghan, Donegal, Carlow, Kildare, Kilkenny, Laois, Longford, Louth, Offaly, Westmeath, Meath, Galway, Leitrim, Roscommon, Sligo, Clare, Cork, Kerry, Limerick and Tipperary.
Wicklow was also added to the list of counties on Monday afternoon.
A similar warning has also been issued by the Met Office for Armagh, Fermanagh, Tyrone and Derry.
A Met Eireann spokesman said: "Scattered thunderstorms developing through the afternoon and evening with heavy downpours and a risk of localised flooding."
This is in addition to a Status Orange rainfall warning issued for Longford, Cavan, Leitrim and Roscommon.
The national forecaster is predicting more than 50mm of rainfall and flooding for these counties throughout Monday afternoon.
The warning is valid from noon until 6pm on Monday.
A spokesman said: "Intense thundery downpours today, in excess of 50mm, will lead to surface flooding."
Parts of the country may see high temperatures on Monday afternoon, but some areas are to experience flooding due to heavy downpours.
A forecaster said: "A dry and mostly sunny start today with mist and fog in the northwest soon lifting.
"Showers will develop by lunchtime and will be heavy and thundery at times, bringing a chance of localized flooding.
"Showers will be most frequent over the midlands and north, and later in the west. Highest temperatures between 20 and 23 degrees, a little cooler along coasts."
They added: "Tonight will be mainly dry for much of the country with showers dying out. Mist and fog patches will develop in light variable or northerly breezes.
"Towards morning showers will move on to the southeast coast. Lowest temperatures of 9 to 11 degrees."
