Where did celebrating St. Patrick's Day with green beer originate?

Not in Ireland anyway. Nobody would dream of fouling good beer with green dye, and since lots of people drink Guinness, which is black, it would be pretty hard to do anyway.

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This is purely American in origin. Some person, most likely not Irish at all decided to desecrate beer in the name of St. Patrick. Real classy. Then with plenty of help from slimy news types it took off. Now it's apart of American holiday tradition. Which is of course, take something and change it into something opposite of what it use to be. And it helps when there is a excuse to drink. Any excuse to drink is the sure fire bet into American tradition. Like football and nascar.

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Not necessarily purely American. Several years ago in Poland, I was out with a friend who ordered a beer (a lager) plus a small glass of Curacao. He poured the Curaco into the beer, producing a slightly sweeter and GREEN beer. Quite a nice drink -- don't remember what he called. it.

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That's an American tradition. You'll find that most Irish people don't like the concept of turning lesser American beers green and calling it Irish. If you want to properly celebrate St. Patrick's, find a good Irish beer you like and raise a glass.

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An origin I heard of was the Irish celebrate St Paddy's day with so much fury that their stock of beer is depleted having them resort to drinking "green beer". A term to brewers meaning beer that is not ready to drink.

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Sadly, this is most likely an American invention. It is another example of how many of my fellow Irish-Americans are completely clueless about their own ancestry and have turned it into a cheap and cartoonish industry of Leprechauns, Shillelaghs and Green things. Half of them are probably Scots-Irish and don't understand the difference.