The Status Yellow warning is in place for all counties in the Shannonside Northern Sound Region.
Cavan, Monaghan, Leitrim, and Roscommon, as well as Sligo, Tipperary, Donegal, Waterford and all of Leinster.
Heavy showers and thunderstorms are expected across the 19 counties.
The alert is valid until 3am tomorrow.
Torrential rain may lead to dangerous driving conditions, according to Met Eireann.
Cathal Nolan from the Ireland Weather Channel says the unsettled conditions looks set to continue, with the long term weather fairly uncertain.