Forecasters say temperatures could hit 21C or even 22C on Thursday.
There should be clear blue skies for most of the day, with the best of the weather in the afternoon, says BBC NI.
The north and west of the county will also see the best of the afternoon temperatures.
Cloud will return on Friday so forecasters are recommending you make the most of the brief return of good weather.
It comes after a wash-out summer for Donegal and below average temperatures.
Temperatures will fall back to around 15C or 16C on Friday but winds will be light – and we should escape any rain.