Peter’s Two Dads is episode 10 of family guy season 05. It was originally aired on February 11, 2007.
After Peter’s dad, Francis, dies, he finds out from Thelma that his real father lives in Ireland. So Peter and Brian head there in search of his father, who ends up being the town drunk, which is a great honor, and he challenges him to a drinking game to prove his paternity. Meanwhile, Stewie acts out and Lois spanks him, but he quickly realizes that he likes it and does whatever he can to get into trouble.