People have been getting into costume in Londonderry and Belfast
Thousands of people have been attending St Patrick's Day celebrations across Northern Ireland.
In Belfast, a carnival procession started proceedings, with X-factor star Eoghan Quigg headlining a concert at Custom House Square.
In Londonderry, events included a free traditional Irish music concert at Victoria Market, and a parade featuring local community groups and performers.
Dublin staged the largest parade on the island, with up to 500,000 people.
They watched more than 2,000 performers from street theatre companies and marching bands who had travelled from as far afield as the US, Canada, Germany, Italy and Bulgaria.
Belfast Lord Mayor Tom Hartley said that St Patrick's Day has become one of the highlights of Belfast City Council's events calendar.
"The council has worked extremely hard to make the event fully inclusive, so that it appeals to all sections of the community, and this work definitely has paid off, to make St Patrick's Day an occasion which can be enjoyed by all the people of Belfast, and visitors alike," he said.
The Princess Royal presented the Irish Guards with shamrock
There have been celebrations all over the world, with Irish regiments in the British Army marking the saint's day.
The 1st Battalion Irish Guards held a St Patrick"s Day parade held at Victoria Barracks in Windsor, Berkshire, where they were presented with shamrock by the Princess Royal.
Patrick was captured by Irish raiders from his home in Wales and reputedly herded sheep on the slopes of Slemish in County Antrim during his captivity.
He managed to make his way home after six years, but later returned to Ireland with a mission to convert the Irish to Christianity.
Legend has it that he used the shamrock to teach the concept of the Trinity and banished snakes from the island.
It is unlikely there ever were any snakes in Ireland, but the snake story may be a reference to serpent, a symbol of evil, and the driving out a reference to Patrick's mission to rid Ireland of pagan influence.
St Patrick's Day is held on 17 March, which is believed to be the day he died.