Uncle O'Grimacey is Grimace's Irish uncle. He has the same general morphology as the other members of his family. Unlike his nephew, O'Grimacey's hair is green, thicker, and longer. He wears a green hat and shamrock-print vest, and holds a short shillelagh.
If you hadn't guessed, his appearance and role in McDonaldland are examples of the sort of Irish caricatures and stereotypes that have come to attend St. Patrick's Day.
He first appeared in 1978 print and TV ads, where he was voiced by Lennie Weinrib, for the McDonald's Shamrock Shake. He was phased out sometime during the 1980s, for reasons I have yet to discover.