Sláinte is the traditional greeting today, and the normal toast is made with a pint of a certain black stout. More extreme fans of St Patrick's Day – as found among those of Irish descent in the US – will be dressing in as much green as they can lay their hands on, painting their faces and even dyeing their hair.
This year, in celebration of the Celtic saint's day, more than 40 international landmarks are being lit in green. From the pyramids and the Leaning Tower of Pisa, to the Sydney Opera House and South Africa's Table Mountain: some of the most recognised man-made and geographical attractions will join a host of British landmarks to "go green".
In the UK, the London Eye, HMS Belfast, Selfridges department store and Liverpool FC's Anfield stadium will all turn a four-leaf-clover shade of green as part of the celebrations.
Other landmarks around the world will adopt what the Irish tourist board calls "global greening", including the Burj tower in Dubai, Niagara Falls, and the entire Costa Blanca town of Moraira.
In Dublin, up to 8,000 visitors from around the world marched in a "people's parade" yesterday. The Irish Taoiseach, Enda Kenny, meanwhile, joined a procession through New York. Mr Kenny and President Barack Obama are scheduled to meet at the White House on Tuesday for a traditional exchange of shamrocks.
The newly elected Pope Francis sent a message to the people of Ireland to mark the day, greeting Michael D Higgins, the Irish President, and commending the nation "to the powerful intercession of St Patrick".