Where did this 'irish curse' myth originate?

Irish people are the product many dif. people gaels, picts, vikings, normans, english, so why would the size be significantly different. Also, Irish people are amongst the taller people in europe and the world so proportionately they should be the same(not strictly true, but anyway). I ume it began in america as a racial slur but what are your thoughts? btw I myself am 7.5 inches erect