Well, it is and it isn't. Certainly, St Patrick was considered Irish, although he was an immigrant from Britain sometime around 450. He was named Ireland's patron saint long afterward and his feast day was established in 1631. Still later, his feast day became a break in Lent (which is a very long period of abstinence!)
But the celebration we now know was developed by Irish immigrants to New York and Boston and embellished even further in Chicago. It was a way to maintain their identity and solidarity. By the 1900's Americans were wearing green, eating corned beef and cabbage, and holding huge parades. It wasn't till 1904 that Ireland named it a holiday, following their American cousins.
So why the green clothes? Green was the colour of the flag of the Great Irish rebellion against Britain in 1641. And why is the meal of the day corned beef and cabbage? Immigrants to New York found corned beef a cheaper alternative to the traditional salt pork and cabbage is always a cheap veg. And the parades are a sign of the solidarity of the immigrants.
Oh! and the Illinois EPA says that the green stuff that gets poured into the Chicago River on this day is a non-toxic vegetable dye. SLAINTE!
