A yellow weather warning has been issued for 15 counties in the Republic and the six counties in the North.
The rain and thunderstorm alert from Met Éireann will cover all of Leinster, in addition to Donegal, Cavan and Monaghan.
The warning will take effect at 12pm on Saturday, lasting until 8pm.
Valid: 12:00 Saturday 21/08/2021 to 22:00 Saturday 21/08/2021
Issued: 10:06 Friday 20/08/2021
Yellow - Thunderstorm Warning for Antrim, Armagh, Down, Fermanagh, Tyrone, Derry
UK Met Office Weather Warning (https://t.co/BFu6qGik8M) pic.twitter.com/c8ckazeUT1
— Met Éireann (@MetEireann) August 20, 2021
A similar warning has also been issued by the UK Met Office for the North, lasting from 12pm to 10pm on Saturday.
Met Éireann are predicting "scattered heavy showers with localised thundery downpours". The conditions will bring a risk of spot flooding in parts.
Meanwhile, the UK Met Office warns "heavy showers and thunderstorms may cause some transport disruption and perhaps flooding".
The weekend's forecast is not looking much better for areas not including in the warning however, with scattered showers due to blow in from the west throughout Saturday.
Conditions are due to improve slightly on Sunday, starting off overcast but skies will clear to bring some brighter, sunny spells on Sunday evening.