Met Éireann's weather station at Dublin Airport recorded 11.5mm of rain in just one hour between 5am and 6am.
That is more rainfall than it saw in the whole month of May.
Meteorologist Liz Walsh says the east of the country has fared worst today in terms of rain.
“A line of heavy showers came in off the Irish Sea.
“And so we’ve got about 21mm so far in Dublin Airport and Casement, looking at a further up to 10mm for the rest of the morning.”
Meanwhile, a status yellow rainfall warning has been issued for seven counties until 2pm today.
The warning is in place for Dublin, Kildare, Longford, Louth, Meath, Cavan and Monaghan.
Met Éireann said there will be "further heavy downpours moving westwards with spot flooding".
Later today there will be bright and sunny spells “but further showers or thunderstorms will occur”.
Met Éireann says that highest temperatures will be between 17 to 20 degrees generally but a little cooler in coastal parts of the west and north.
Tomorrow will start cloudy but bright spells will emerge.
The forecaster said temperatures could hit 21 degrees but warned: “scattered showers will occur and will be most frequent in Leinster and Ulster with some heavy bursts”.
The rest of the week is looking “changeable” but Met Éireann said is likely to become breezy on the weekend.