Met Éireann has issued a status yellow rainfall warning for 20 counties, as communities along parts of the River Shannon prepare for possible flooding.
The weather warning comes into force from 8pm and will be in effect until 8am tomorrow morning.
It covers Connacht, Cavan, Monaghan, Donegal, Dublin, Kildare, Laois, Longford, Louth, Wicklow, Offaly, Westmeath, Meath, Limerick, Clare and Tipperary.
Up to 25 millemetres of rain is expected to fall in parts of the country overnight.
Forecasters say the rain is likely to be preceded by sleet and snow in areas of Connacht, Ulster and north Leinster.
The ground in many areas is already saturated, while river levels are also elevated - meaning there is a risk of localised surface and river flooding tonight.
Flood defences are in place along parts of the River Shannon ahead of the expected heavy rain, while some farmland areas around Athlone and Carrick-on-Shannon are already underwater.
Mayo County Council spokesperson Noel Gibbons, meanwhile, said there is already flooding on some roads in the county.
The flooding on the R334 at the Neale crossroads has increased & road is impassable.
It is fully closed with no local access & diversions remain in place. South bound traffic from Ballinrobe please divert via N84 Kilmaine. North bound traffic divert in Headford via Kilmaine. pic.twitter.com/j9MS0g68ba
— Mayo County Council (@MayoCoCo) February 23, 2020
He said: "With more rain promised over the next few days, these water levels will rise - so we're expecting more roads to be flooded over the coming days.
"We're [asking] motorists not to drive through flooded roads - in particular roads that have been closed."