Temperatures rose towards the all-time March high record of 21.7C recorded in Armagh in 1965, hitting a hot 21C in Ballykelly in the north west.
There were also highs of 20C on the north and west coasts, although it was cooler by the sea where the mercury hit a more subdued 13 or 14C.
Ballykelly saw temperatures warmer than more southerly parts of continental Europe, including Barcelona, Nice, Majorca and Faro in Portugal, none of which saw temperatures rise out of the teens Celsius yesterday.
Out in the fresh air a group of sporty lads in shorts and T-shirts were playing an energetic game of volleyball in Portrush under a bright blue sky.
Others took the perfect opportunity to visit amusement parks while the weather was good — prompting plenty of family fun in the great outdoors.
And there was lots of flashing of the flesh in Belfast city centre and beyond as people took the opportunity to unveil their summer wardrobes earlier than expected.
Supermarkets and shops were doing a roaring trade in ice-creams, cold drinks and throw-away barbecues as the sun continued to shine.
Although the clocks went forward on Sunday, depriving us of an hour in bed, there were few complaints when it really did seem like summertime.
Meteogroup forecaster Claire Austin said the balmy weather is here to stay for the next few days.
“The sheltered inland areas of Northern Ireland seem to be a bit warmer than the coast, where it’s much cooler,” she said.
“On the east coast we’re only at 13-14C because there’s a bit of a wind off the sea keeping the temperatures down.
“But when you get to the sheltered western parts of Northern Ireland and central areas, that’s where you’re seeing the warmer temperatures.”
Ms Austin said that yesterday’s sunshine wasn’t the start of a heatwave, but she also confirmed that currently there is no rain on the horizon.
“Basically we’ve just got high pressure at the moment so we’re getting some nice warm temperatures,” she said.
“Temperatures are going to stay pretty similar throughout the week, but it doesn’t look like it’s getting any warmer,
“If anything, by the end of the week it’s going to start getting cooler again, but there doesn’t seem to be any hint of rain for this week, or into the weekend.”
Other parts of the UK also basked in glorious spring sunshine yesterday, although Scotland recorded the highest temperature of 22.2C at Fyvie Castle near Aberdeen in the north-east of the country.
But other parts of the North Sea coast would have been hard pressed to notice it was spring at all, with lingering fog keeping temperatures down to around 6C in East Anglia and Kent.