Season 5: Episode 87: Peter's Two Dads
After Peters dad, Francis, dies, Thelma tells him that his real father actually lives in Ireland. So Peter and Brian decide to head there in search of his father, who ends up being the town drunk. They decide to play a drinking game to prove his paternity. Meanwhile, Stewie acts out and Lois spanks him. Realizing that he likes being spanked, Stewie does whatever he can to get into trouble.