A status orange thunderstorm warning is in place for four counties on Monday.
It affects Donegal, Leitrim, Mayo and Sligo with Met Éireann warning of the potential for flash flooding and very difficult travelling conditions.
The forecaster also says there could be damage to power lines.
It will run from 2pm until 9pm.
There's a similar status yellow alert in place for eight counties until the same time in the rest of Connacht, Donegal, Longford and Cavan.
The sunny weather will come to an end in the afternoon, with heavy showers and a chance of lightning.
The showers will continue through to this evening, and it will turn more overcast too. Highest temperatures of 19 to 23 degrees and light winds.
Met Éireann is warning of the possibility of flooding and difficult travelling conditions.
The Road Safety Authority are warning drivers to slow down and allow a greater braking distance between themselves and the vehicle in front in wet weather conditions.
Road users are also advised to find alternative routes if a road is flooded, and to drive slowly with your foot on the brake when going through water.
Pedestrians are advised to wear visibility clothing and walk on the right side of the road.
A yellow thunderstorm warning has also been issued for Tyrone, Derry and Fermanagh from 1pm until 9pm.
Showers will continue overnight, with the lowest temperatures of 10 to 13 degrees.
