I won't by no means indicate, that I'm a knowledgeable person, but googling around I found the following link
http://www.irish-poteen.com/tradepress_harpers3.html
In short, poteen seems to be a triple-distilled grain or potato spirit. It comes in three strengths 60%, 70% and 90% ABV. Sounds to me like a german "Korn" though. Now I can imagine, why "apparently, the Church was not amused"

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