It is a public holiday in the Republic of Ireland, Northern Ireland, Newfoundland and Labrador and in Montserrat. It is also widely celebrated by the Irish diaspora, especially in places such as Great Britain, Canada, the United States, Argentina, Australia, New Zealand, and Montserrat, among others." From Wikipedia
Patrick lived lived in the region of the former capital of Ulster at Emain Macha and at Ard Macha, the present day Armagh, which still claims descent from him. King Daire held court here and St. Patrick set up his church within the royal rath. It grew into a monastery which is now the site of the cathedral.
As a consequence of his challenge to Druidical authority, a trial of powers was arranged.
According to legend, Patrick put forward a scheme which seemed to favor the Druid. A hut was constructed in two sections, one half of dry wood and the other half of green wood. Lucetmael unknowing stood inside the green part wearing the saint's cloak while Benen, a follower of Patrick's, entered the dry half wearing the stolen mantle of the Druid. The shelter was set on fire. Lucetmael was completely consumed in the flames. The Druid cloak burned only on the outside to ashes.
Depicitions of the Saint generally show him with snakes at his feet, recalling the legend that he banished all the evil, from the land.
The Columbia-Viking Desk Encyclopedia Third Edition The Viking Press 1953-revised
The Celtic Book of Saints Courtnry Davis text by Elaine Gill Blandford Book 1995