Bennigan's is a real chain of restaurants. It first appeared in the Season Five episode, "Butters' Very Own Episode". It was seen again in "Professor Chaos" and was subsequently mentioned in "Canada on Strike".
Background
Butters has an extreme fondness for Bennigan's. During "Butters' Very Own Episode", his parents planned on taking him there for their wedding anniversary. Later, Butters is seen singing "Happy Anniversary from Everyone at Bennigan's", while receiving a ride from a truck driver.
Inside the restaurant.
In "Professor Chaos", he switches two soup orders, in a futile attempt to cause chaos as his villainous persona.
In "Casa Bonita", Cartman fools Butters into thinking he is the sole survivor of an apocalypse and leads him to a dump, where he creates his own civilization, including buildings made out of garbage. One of the buildings, Butters identifies as a Bennigan's, once again requiting his love for the restaurant.
Bennigan's was mentioned in "Canada on Strike". After a long "strike", the Canadians finally win their benefits, only to receive free coupons to Bennigan's and bubble gums for each Canadian.