Met Éireann has issued Status Orange and Yellow weather warnings for heavy rain and flooding that impacts several counties.
The orange alert, which covers counties Cork and Kerry, came into effect at 3am and is valid until 6pm.
The forecaster is predicting "significant rainfall accumulations" - especially on high ground.
It said the highest totals will occur in Co Kerry and in west Cork.
Met Éireann said it expects significant river and surface water flooding in the impacted areas, which may lead to hazardous conditions for travel and poor visibility.
Status Orange weather conditions may pose a threat to life and property.
A Status Yellow rain alert is in effect for counties Clare, Cork, Kerry, Limerick, Donegal and all of Connacht until midnight.
Another yellow weather warning for counties Tipperary, Waterford and Wexford is in place until 3am tomorrow morning.
In these counties, Met Éireann said the flood risk is increased "due to high river levels and saturated ground".
The UK Met Office has issued a Status Yellow alert for counties Antrim, Armagh, Down, Fermanagh, Tyrone and Derry.
The Met Office said periods of heavy rain may lead to travel disruption and some flooding.
The advisory is in place until 6am tomorrow morning.
The Road Safety Authority has appealed to road users to take extra care for the periods that the weather warnings are in place.
