Met Eireann has issued a grim forecast as hazardous conditions are expected in less than 24 hours.
Temperatures of 21C have shone on Ireland on Saturday before rain buckets down on Sunday.
A polar maritime airmass will move across Ireland on Sunday bringing rain, wind and cooler conditions.
Meteorologist, Andrew Doran-Sherlock, added: "This weekend will see the last of the above average temperatures, with some places reaching around 20 degrees Celsius.
"However, from Monday we'll see a significant change with a much more autumnal feel to our weather, as temperatures dip to a cooler 12 to 15 degrees Celsius.
"Along with lower temperatures, it'll turn quite unsettled too with spells of rain and heavy showers along with some windy conditions, which current indications suggest will persist through much of next week."
On Saturday night, scattered patches of light rain and drizzle will continue but there will be some clear spells also
An “unsettled” spell of weather is set to move in from the Atlantic from Sunday onwards.
Met Eireann say rain will develop in the west and southwest on Sunday morning and gradually spread eastwards through the day, bringing some heavy downpours and the risk of spot flooding.
However, some eastern areas will stay dry until late on Sunday afternoon.
It will be noticeably cooler from the west as the rain clears.
And it will continue “unsettled” for the rest of next week as some wet and windy weather is on the cards.
A Met Eireann forecaster said: "Becoming noticeably cooler and more unsettled in the coming week, with temperatures falling back to around average and heavy showers or spells of rain at times."
"Monday will be a much cooler and fresher day with a mix of sunny spells and heavy showers. Showers will be most frequent over the western half of the country with the potential for thunderstorms.
"Highest temperatures falling back to around normal for the time of year at 12 to 15 degrees."
Tuesday will be a wet day with widespread heavy showers or longer spells of rain developing again.
The heaviest showers will gradually clear to the north on Tuesday night, leaving a mix of clear spells and scattered showers.
It will be another showery day on Wednesday, with showers feeding in across the country.
The weather is will remain generally unsettled into next weekend with spells of rain and heavy showers at times.
It will be staying cooler too, with temperatures in the low to mid-teens.
