The Orange rain warning was issued for Carlow, Kildare, Kilkenny, Laois, Longford, Offaly, Westmeath, Cavan, Donegal, Leitrim, Roscommon, Sligo, Waterford and will last from 4am-8am on Sunday morning.
The national forecaster said up to 50mm of intense rainfall will hit the affected counties, including Co Laois, where thousands are camping at the Electric Picnic festival this weekend.
A Status Yellow rain warning will also take effect from 1am on Sunday and will last until 10am and covers the entire country.
In an update issued on Saturday, the national forecaster said that rain will be torrential at times in localised bursts and that this would result in surface flooding in affected areas.
Met Éireann had previously warned of disruption due to the inclement weather, including for the 70,000 people who are attending Electric Picnic in Stradbally, Co Laois, this weekend.
There is also the likelihood of lightning strikes and thunderstorms along with very heavy rain at times on Saturday night and into Sunday.
“There will be clear spells and scattered showers at first, but more persistent showery rain, accompanied by local thunder and strengthening southeasterly winds, will then sweep northwards over the country. The rain will be very heavy in places, bringing a possibility of localised flooding with some lightning strikes,” said forecaster Conall Ruth.
“Heavy rain will move northwards across much of the country tonight, clearing from the north and northwest on Sunday morning. Some torrential downpours are expected in parts, resulting in surface flooding. There is also the potential for thunderstorm activity and lightning strikes,” Met Éireann said in an updated yellow weather warning on Saturday morning.
Heavy rain, along with thunder and lightning will clear to the northwest Sunday morning, but southeasterly winds will remain very blustery in parts.
Sunday afternoon and early evening will be largely sunny with just isolated showers before a further band of heavy rain will push in from the southwest.
The weather is forecast to be very unsettled with wet and windy conditions predominant next week.
Early next week, the weather is likely to be mixed with scattered showers and sunny spells on Monday and Tuesday.