Met Éireann has issued a Status Yellow thunderstorm warning for nine counties.
The alert, covering counties Cavan, Donegal, Monaghan, Leitrim, Roscommon, Sligo, Longford, Offaly and Westmeath, is valid until 9pm on Monday.
The forecaster predicts scattered thunderstorms this afternoon and evening with localised downpours leading to poor driving conditions.
A Status Yellow thunderstorm warning is also in place for Northern Ireland. The warning covers counties Antrim, Armagh, Down, Fermanagh, Tyrone and Derry and is valid until 10pm tonight.
Meanwhile, showers will clear in the north early tonight, becoming largely dry with clear spells.
However, further showery rain will spread from the southwest overnight, turning heavy at times.
It will become blustery as the rain moves in with moderate to fresh and gusty southeasterly winds developing, increasing strong along Atlantic coasts.
It will be a mild night with lowest temperatures between 10C to 12C.
Tuesday will be cloudy across the eastern half of the country with some rain or drizzle in parts in the morning.
Persistent rain will spread across Leinster and east Ulster from early afternoon, turning heavy at times.
The west will have a largely dry day with some sunshine at times, however a few showers will develop in the afternoon that may be heavy or thundery.
There will be highest temperatures between 14C to 17C in moderate to fresh southerly winds, strong for a time on western coasts.
It will remain unsettled for the remainder of the week with spells of rain and showers. Temperatures will return closer to average, and it will be blustery at times.
A band of showers will move northwards across the western half of the country on Tuesday night, becoming heavy at times.
It will be mostly dry in the east with isolated showers. It will be breezy, windiest near the west coast where it will be very blustery. Winds will be coming from a mild southerly direction with lowest temperatures of 7 to 10 degrees. Turning drier towards morning.
Wednesday will start largely dry and sunny with isolated showers in the northwest.
However, a band of rain will move in from the southwest and track northeastwards across the country through the afternoon and evening with some heavy downpours possible.
Highest temperatures will be between 14C to 18C in moderate to fresh and gusty south or southeasterly winds, increasing strong at times near coasts.
Rain will clear the northeast on Wednesday night, with long clear spells developing.
Some mist or fog patches may form as the winds largely ease, although fresh to strong southwest winds will persist on western coasts with lowest temperatures between 6C to 8C.
This comes as yesterday was the warmest day of the year so far, according to Met Éireann, with one weather station in Connacht reaching a temperature of 20.9C.
In a social media post last night, the forecaster said Mount Dillon in Co Roscommon saw the highest temperature.
It added that four other weather stations reached at least 20C yesterday. They include Shannon Airport in Co Clare, Belmullet in Co Mayo, Mace Head and Athenry in Co Galway.
The previous warmest day was 27 March, when Met Éireann reported 18.9C in Newport Furnace in Co Mayo.
Last year's highest temperature was also reported at Mount Dillon on July 21, with a temperature of 30.8C.