The yellow weather warning for radiation fog today is effective in Northern Ireland, Wales and parts of England until 11am.
While Limerick city and county is now enduring dense fog at present, Met Eireann expects this to worsen between Thursday evening and the early hours of Friday morning.
They said: "Heavy rainfall expected later Friday, overnight and into early Saturday with falls of 30 to 50mm expected".
While fog will begin to clear in the afternoon, it will linger locally in places ahead of a chilly day in most of the country, although it will be slightly milder in southern counties, where temperatures will reach highs of 8 degrees. This will become more extensive again after dark.
The meteorological office expects rather windy conditions tomorrow, with strong and gusty southeasterly winds given.
Friday is set to be breezy followed by heavy rain in Munster or Connacht. Afternoon temperatures will range from 5° to 9°, while it will be coldest in the north.
It will be windy and mostly cloudy elsewhere too with the rain spreading eastwards; some heavy rain is expected by dawn.
Britain is wrapping up for another chilly night with temperatures expected to drop to freezing widely with lows of -2C (28.4F) or -3C (26.6F) in rural regions. Top afternoon temperatures of 3° to 7° are given for the day.