It's when a guy takes a full beer (preferably Guinness) and empties it in a womans vagina during intercourse. The man then proceeds to drink the alcohol from the opening as it flows out in similarity to a waterfall.
An Irishman is having sex with a lady, and decides to pour his beer right down her vagina in order for him to go drink, and lick it out while pleasuring his companion. This is then called an Irish Waterfall (Irish fish market, Luck of the Irish, Drunken Irishman).
by GregoryLarkinRonan March 18, 2010