A status-yellow thunderstorm warning is now in place for 23 counties, with heavy downpours and a risk of hail and spot flooding expected.
The Met Éireann alert came into effect at 11am and will remain in place until 9pm.
The warning applies to all counties in Connacht and Munster, Carlow, Kildare, Kilkenny, Laois, Longford, Louth, Meath, Offaly, Westmeath, Wexford, Cavan and Monaghan.
Scattered thunderstorms are forecast throughout this afternoon and evening, bringing widespread, heavy showers, mostly frequent in the midlands and the south of the country.
Localised thundery downpours will bring a risk of spot flooding, the forecaster said.
Dublin, Wicklow and Donegal are the only three counties in the Republic not impacted by the weather warning, with Met Éireann also predicting isolated showers along the east coast, while the northwest is expected to experience the best of Sunday's dry and bright weather.
Highest temperatures of 13 to 16 degrees are forecast for today, dropping to lows of two to six degrees overnight.
Dry and mostly clear weather will extend from the north to the south early on Sunday night as showers die out, except for a few isolated ones on northern and western coasts.
Monday will see a dry and sunny start to the week, Met Éireann said, before showers develop later in the morning.
However, the showers are not expected to be as heavy or frequent as they were during the weekend.
Met Éireann has also predicted plenty of sunny spells later in the afternoon on Monday, with a largely dry evening expected.
Temperatures will be slightly cooler with highs of 11 to 14 degrees expected.