Met Eireann is warning of heavy and persistent rain from tonight until Friday morning, with a status yellow rainfall warning in place for Connacht and 5 other counties along the western seaboard.
The forecasters says the heavy rain will lead to some flooding and disruption.
Up to 60 mm of rain is expected across the region - higher in mountainous areas.
The warning comes into effect from midnight tonight until 6am Friday morning.
There are also gale warnings and small craft warnings in place today.