At least 30 families have been evacuated from their homes in the Co Laois town of Mountmellick.
It comes after the Owenass River burst its banks earlier.
The flooding is affecting two housing estates in the town, and local authorities are urging drivers to avoid the area.
In a statement, the council said: "Roads crews, Civil Defence crews and Fire Service crews are in attendance.
"Members of the public are requested to observe road closure signs, drive appropriately and not attempt to drive through flooded areas."
Teams assisting with the evacuation of people from their homes due to flooding in Mountmellick at the moment. pic.twitter.com/fw3AT4hdFN
— Laois Civil Defence (@LaoisCivilDefen) November 22, 2017
Flooding around Co. Laois- all crews currently mobilised. pic.twitter.com/oVg1nyMKBn
— Laois Fire & Rescue (@LaoisFireRescue) November 22, 2017
The council is also warning residents in the Portarlington area that water levels in the River Barrow "are high as a result of the recent heavy rainfall and are expected to rise further over the next six hours".
Cllr Padraig Fleming is Cathaoirleach of Laois County Council, and he explained: "If the houses are flooded... you know the effect of flooding.
"We've seen it throughout Ireland in various places. It destroys houses."
He said flooding has hit Mountmellick this evening in an "unprecedented way".
A status yellow rainfall warning remains in place for Munster and Leinster until 10pm tonight.
Many parts of the country experienced heavy rain all day, leading to major delays on roads like the N3 and M4.
Earlier today crews from Tallaght responded to a housing estate flooded with several houses about to flood in #Newcastle. Along with crews from @sdublincoco the water was prevented from breaching the houses and pumped/drained away. pic.twitter.com/d9dp8hjTgn
— Dublin Fire Brigade (@DubFireBrigade) November 22, 2017
N3 Eastbound (towards city centre) is now closed from Junction 4 Clonee due to flooding. Please divert to N2/N3 linkroad or alternative route. There is extreme traffic conjestion in the Dublin 15 area. @mickcarroll28 @aaroadwatch @dublinlive pic.twitter.com/dCl0AodaPp
— FingalCountyCouncil (@Fingalcoco) November 22, 2017