Parades and festivities have taken place across the country to mark Ireland's national holiday.
In Dublin, half a million people, including more than 100,000 overseas visitors, are estimated to have attended the city's St Patrick’s Day parade.
Other parades took place in cities, towns and villages across the country, while celebrations are set to continue long into the night.
Around 80,000 people are estimated to have attended the Limerick parade, with tens of thousands lining the streets in Cork, Waterford, Galway and many other locations.
A larger than normal garda presence is visible, especially in the capital, to clamp down on anti-social behaviour and to help deal with the massive crowds.
These American tourists spoke about what they know of Ireland's patron saint:
Following 18 months of preparation, the Dublin parade's theme 'Imagine If...' looked to the next 100 years with around 3,000 Irish and international participants putting on a futuristic show.
Backstage at St Patricks parade in Dublin - plenty of practice going on, not a bum note in the house! #StPatricksDay pic.twitter.com/lH4hFGB9aE
— Sean Defoe (@SeanDefoe) March 17, 2016
The Berkeley police and fire department marching in this year's Dublin parade following last year's tragedy pic.twitter.com/E6sl7UIrzg
— Sean Defoe (@SeanDefoe) March 17, 2016
The Grand Marshall of the Dublin parade was disability rights activist Joanne O'Riordan, who said she was 'delighted' by the honour.
Speaking ahead of the parade, CEO of the St Patrick's Festival Susan Kirby said marching bands from all over the world flew in to be part of today's festival:
In Galway, off licences and supermarkets agreed not to sell alcohol before 3pm.
The agreement with the gardaí for St Patrick's Day was voluntary and a move to prevent anti-social behaviour on the city's streets.
Lord Mayor of Galway City Councillor Frank Fahy praised the arrangement, saying that it would cut down on the problem of public drunkenness.
St Patrick's Day celebrations in Limerick, where 80,000 people are attending
Road users are being urged to drive with extra care this evening as they make their way home from the festivities.
The RSA says the roads are expected to be busier than usual and is reminding people to never drink and drive.
Brian Farrell, Communications Manager with the RSA, is advising drivers to be on the lookout for vulnerable road users given the amount of people out enjoying today's celebrations and good weather:
Meanwhile, 415 Irish soldiers will be spending St Patrick's Day on peace missions in 15 countries.
The peace support missions include Mali, Kosovo, Lebanon and The Democratic Republic of Congo.
The troops will celebrate St Patrick's Day with a mix of military and cultural traditions.
Captain Ross O’Shea will be spending the big day in the Golan Heights. He tells us what he's doing there:
