Northern Ireland and Scotland have both recorded the most sun in March since records began.
The UK as a whole recorded the sunniest March since 1929, according to the Met Office.
Northern Ireland basked in 90% more sunshine than average for the month, with a total of 192.5 hours of sun.
Scotland saw 64% more sun than usual, with a total of 160.1 hours.
The sunny outlook was replicated across the UK, with England seeing its second sunniest March at 168.1 hours and Wales its fifth sunniest with 157.9 hours.
Met Office records on sunshine hours first began in 1919.
Mark McCarthy, head of the forecaster's National Climate Information Centre, which manages the UK's climate records, said large areas of high pressure which settled over the UK for much of the month caused the record-breaking figures.
He said: "March has again illustrated the typical variability of the UK's climate, with some cold conditions at the start and end of the month separated by a long spell of mild and settled weather.
"The record-breaking sunshine figures were largely brought on by a very large area of high pressure that sat over the UK for much of the month, at times also covering much of Europe.
"This brought clear skies, mild days, but also some cool nights and some frosts.
"We've seen the warmest weather of the year so far with 20.8C recorded in London on March 23, but we also saw a low of minus 9.1C at Aboyne (Aberdeenshire) at the start of the month, highlighting the variability of March in the UK."
The high pressure also kept wind and rain to a minimum in March, which was in contrast to February when three named storms hit the UK in just a week.
In March, the average maximum temperature for the month was 11.1C - some 1.9C above average.
This places the month as the sixth mildest March for maximum average temperature since the series began in 1884.
Scotland also saw its sixth mildest March by maximum temperature with an average 9.8C, Wales its fifth with 11.4C, Northern Ireland its seventh with 11.2C and England its ninth with 11.9C.
The balmy weather peaked between March 23 and 27, with 19C reached around Carlisle, Glasgow and Inverness, and 20C locally in parts of England and Wales.
Most of the month stayed dry, but spells of rain at the start and end of the month prevented it reaching any record levels.
It was especially dry in the north of the UK, with Scotland seeing only half of its usual rainfall, at 61.9mm.
The UK as a whole also had 58% of its average rainfall, with 46.9mm.