Met Éireann has issued a status yellow weather warning as up to 40mm of rain is forecast for today. Winds may also reach gale force.
Rain will move southeastwards across the country during the morning and early afternoon and will be heavy in places.
Dublin, Carlow, Kilkenny, Wexford, Wicklow, Cork and Waterford will all be hit, with spot flooding likely in places.
Brighter weather, with just a few scattered showers, will follow across Connacht and much of Ulster and a few dry spells may develop in other parts.
However, it will be mostly cloudy, with further outbreaks of rain at times in much of Munster, Leinster and east Ulster, and with a risk of localized flooding.
Highest temperatures will be 15C to 17C, with strong southerly winds in places at first veering southwesterly and decreasing mostly light to moderate.
Met Éireann forecaster Joanna Donnelly said: "All parts of the copuntry will be affected today. (The rain) leads off from the west, but the west will also be the ones brightening up first. (The rain) will probably persist in the east of the country for the rest of the day."
Rain in parts of the southeast will clear tonight and many areas will be dry, with variable cloud cover. However, some scattered showers will occur, mainly on the west coast.
There will be light winds inland and mist and fog are expected in places. It will be quite cold, with lowest temperatures of 3C to 7C.