Why does every non-Irish person make the assumption that the irish are either leprechauns or alcoholics? ?

I am irish and most people I meet that aren't from Ireland automatically assume that I am an alcoholic, or in their words "mad for the drink" (usually executed in a horrific attempt at an Irish accent). Also when it goes as far as insults, 'leprechaun' is usually the first word thrown in and in some cases 'paddy'. Why are these stereotypes so widely recognised? Like one third of the American population is obese but when I meet an American or someone of American decent I don't automatically assume they're obese.