Uncle O'Grimacey is a character of the fictional McDonaldland, created for an advertising narrative of the McDonald's fast-food restaurant chain both in celebration of St. Patrick's Day and to mark the annual appearance of the Shamrock Shake. O'Grimacey is the Irish uncle of the character Grimace and is a variant of the Grimace-design in that he is green instead of purple, sports a frock coat covered with several four-leaf clovers, and carries a shillelagh. His design motif is not unlike that of a stereotypical depiction of the Irish folkloric Leprechaun, similar to the mascot of the football team for University of Notre Dame. O'Grimacey resides in his home country for eleven months of the year and visits his nephew Grimace in March, bringing with him his "incredibly delicious" shake. A popular character used annually by the restaurant in the 1980s, Uncle O'Grimacey is no longer used by the chain for its promotions of the shake. However, despite a long absence, McDonald's has brought its annual dessert back in some cities.
To further enjoy your St. Patrick's Day season, you can go to these sites and learn about the history and traditions. Apparently, it's not just about drinking. It seems there is some religious significance as well. Of course, being a drunkard is my religion, so I guess it all works out.