Thousands of people attended the celebrations in Downpatrick, Newry, Armagh, Omagh, Strabane, Enniskillen and Londonderry on Monday.
One of Ireland's largest events took place in Dublin city centre where ornately decorated floats dedicated to the Patron Saint passed along the 1.6 mile route from O'Connell Street to St Patrick's Cathedral, watched by hundreds of thousands of people.
It comes after Belfast celebrated St Pat's on Sunday with a massive carnival parade and concert which Lord Mayor Máirtín Ó Muilleoir described as the city's biggest ever.
Meanwhile hundreds of famous landmarks around the world, including the Great Wall of China and the Rhine Falls, have been lit up green as part of Tourism Ireland's Global Greening 2014.
Around 30,000 people filled the streets of Downpatrick, the town which claims St Patrick as their own, as one of NI's biggest parades made its way from Ardglass Road to Rathkeltair House, following a re-enactment of the saint's journey up the River Slaney.
Dr Tim Campbell, director of the town's St Patrick's Centre, said: "We've taken the politics out of the day, and many other towns are following in our footsteps. We just want people to have a fun day."
In Armagh, over 15,000 people were expected to turn out for a colourful procession starting at Palace Demense and passing along to city centre.
It featured a variety of floats, marching bands and street performers.
Lord Mayor, Robert Turner, said: "There is always an amazing buzz across the City for this celebratory event and it's great to see so many people, young and old get involved."
Omagh's parade and open air concert on Monday came as the highlight of a two week festival. Its theme was 'Connecting Cultures' and it began at Omagh House.
Meanwhile in Derry the streets were filled with costumed revellers as over 30 groups from across the North West joined forces for the St Patrick's Day Spring Carnival Parade.
And thousands lined the streets of Newry for a cross-community parade which encouraged voluntary, youth and school groups to get involved in making floats.
The flagship parade in Dublin was one of the largest in the world and footage was streamed online to a global audience and an Irish diaspora of about 70 million.
President Michael D Higgins said: "Today we celebrate our wider and diverse Irish family, to whom we remain connected by a strong cultural heritage and history.
"Today is a special day for all those Irish communities great and small across the world that come together in a spirit of pride and joy to celebrate their identity and their links of affinity and affection with their homeland of origin."
There was also a huge turnout in Limerick City where about 70,000 people saw the parade.
In England, the Duke and Duchess of Cambridge visited the Irish Guards at their base in Aldershot to mark St Patrick's Day.
William, who is the regiment's Colonel, provided an update on baby son George and said he had started crawling. But he added that there are no plans just yet for another addition to the family.
Meanwhile, Kate presented sprigs of shamrocks to around 300 soldiers on parade and honoured the regimental mascot, Irish wolf hound Domhnall, with his own bunch of the well known plant.