The Met Office said the country will become colder, cloudier and more unsettled from Monday, bringing the risk of wintry weather in the middle of the week.
Temperatures during the day will halve from the highs reached last week and fall below freezing overnight as a cold front pushes down from the north.
It comes after many areas including Northern Ireland basked in glorious sunshine and highs of 20C (68F) last week and throughout the weekend with many, like Olivia Irwin, enjoying the waves and sunshine on Portrush West Strand.
Alex Burkill, a Met Office forecaster, told the PA news agency: “Wednesday will be the most extreme in terms of wintry weather, as a cold stream of air will start to push southwards bringing unpredictable wet and windy conditions, even snow or sleet.
“It will be markedly colder than what we’ve just seen, daytime temperatures falling by half, and some areas will even struggle to hit 8C (46.4F), which will signify the biggest change in conditions.
“Overnight into Thursday and Friday, many areas will fall considerably lower than freezing, towards minus 4C (24.8F) and minus 5C (23F)
“This wintry weather could even push into southern areas of England and Wales, which isn’t unusual, it is normal to have colder weather in April, but it’s far from the sunny, warm days seen last week.”
Sunday began cloudy and chilly for many parts of the country, but was expected to reach highs of 15C (59F) later in the day.
During the evening, cloud will begin to spread eastwards, bringing showers, dense patches of fog and frost.